Assert for many a purpose (5)
I believe the answer is:
claim
'assert' is the definition.
(claiming is a kind of asserting)
'for many a purpose' is the wordplay.
'for' becomes 'c' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'many' becomes 'l'.
'a purpose' becomes 'aim' ('aim' can be a synonym of 'purpose').
'c'+'l'+'aim'='CLAIM'
Can you help me to learn more?
(Other definitions for claim that I've seen before include "Put in for" , "Allege" , "Right; demand this" , "Assert; request" , "Assertion of right" .)