But this is only part of the problem: "leader" is simply far too vague an indication for "ion". You have no idea as to what kind of leader to look for, nor how many letters it might be. This renders any kind of investigation into what it might be utterly futile, leaving you only with 'presume' and 'place' left to hint as to what the solution might be. You could just about get 'posit' as the common ground between these two words, so the clue might just about be ok as a 'filler' where you can be sure that the solver will have got some of the intersecting letters. But as a solvable clue in its own right - I think we've demonstrated it's too vague.
So my pet hate is the use of overly vague indications for components of the solution. Vague definitions are fine: that's rather the point. Words with many different meanings are also fine: setters should ruthlessly exploit these as much as possible. Manley suggests that "point" for any point of the compass, or "many" for any roman numeral is too vague. I think that you can get away with this: there are, after all, only about 16 points, so your search is reasonably restricted. However, Manley seems quite happy with the use of "boy" to mean "DES", "SID", "TIM", "IAN", ... and about 3000 other possibilities which I find inconsistent to say the least. I try and avoid puzzles with boys and girls in because I know I'm not going to get those clues: the Everyman used to be very fond of this kind of thing, but seems to have bucked its ideas up recently.
In the case of Blob's clue, you don't need to stoop so low. Here are two plausible alternatives which I think make for a better clue - although I expect to be alone in this opinion.
Presume one is on location (8) (the 'is' might be dodgy' but we can maybe let this ride - the alternatives destroy the clue)
Fundamental particle gives up right to one place (8)