Maintain mollusc has one (5)
I believe the answer is:
claim
'maintain' is the definition.
(I've seen this before)
'mollusc has one' is the wordplay.
'mollusc' becomes 'clam' (clam is a kind of mollusc).
'has' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'clam' enclosing 'i' is 'CLAIM'.
(Other definitions for claim that I've seen before include "title" , "Allegation" , "Assertion of ownership etc" , "Put in for" , "Assert; demand" .)