Temptation to keep a cool head for cash (5)
I believe the answer is:
lucre
'for cash' is the definition.
(I've seen this before)
'temptation to keep a cool head' is the wordplay.
'temptation' becomes 'lure' (lure is a kind of temptation).
'to keep' is an insertion indicator.
'head' says to take the initial letters.
The initial letter of 'cool' is 'c'.
'lure' enclosing 'c' is 'LUCRE'.
(Other definitions for lucre that I've seen before include "profits" , "ill-gotten gains" , "(Dishonourably gained) money (poetic, literary)" , "financial gain" , "Archaic wealth, may be filthy" .)