Agreement not starting to produce an effect (3)
I believe the answer is:
act
'produce an effect' is the definition.
(I've seen this before)
'agreement not starting' is the wordplay.
'agreement' becomes 'pact' (pact is a kind of agreement).
'not starting' means to remove the first letter.
'pact' with its initial letter removed is 'ACT'.
'to' acts as a link.
(Other definitions for act that I've seen before include "Exploit" , "Take steps" , "Aussie parliamentarians exert influence here?" , "Major division of play" , "Major section of a play" .)