Declare mollusc has one (5)
I believe the answer is:
claim
'declare' is the definition.
(I know that claim is a type of affirm)
'mollusc has one' is the wordplay.
'mollusc' becomes 'clam' (clam is a kind of mollusc).
'has' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'clam' going around 'i' is 'CLAIM'.
(Other definitions for claim that I've seen before include "Assertion; demand" , "Allege" , "Assert one's right to" , "State to be the case" , "Assert; request" .)