Assert that one has many an ambition (5)
I believe the answer is:
claim
'assert' is the definition.
(claiming is a kind of asserting)
'one has many an ambition' is the wordplay.
'one has' 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'.
'an ambition' becomes 'aim' (I've seen this before).
'c'+'l'+'aim'='CLAIM'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for claim that I've seen before include "Assertion; demand" , "Call" , "Cause the loss of (a life)" , "Allege" , "Right; demand this" .)