Former partner with shellfish grabbing one will cry out (7)
I believe the answer is:
exclaim
'cry out' is the definition.
(synonyms)
'former partner with shellfish grabbing one' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' says to put letters next to each other.
'shellfish' becomes 'clam' (clam is a kind of shellfish).
'grabbing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'clam' going around 'i' is 'claim'.
'ex'+'claim'='EXCLAIM'
'will' acts as a link.
(Other definitions for exclaim that I've seen before include "Cry out in surprise, horror etc" , "Cry out (in shock, etc)" , "Shout out" , "Speak suddenly" , "say!" .)