Core of workers in vessel showing affinity (7)
I believe the answer is:
kinship
'affinity' is the definition.
(synonyms)
'core of workers in vessel' is the wordplay.
'core of' indicates the central letters.
'vessel' becomes 'ship' (ship is a kind of vessel).
The middle of 'workers' is 'k'.
'k'+'in'+'ship'='KINSHIP'
'showing' is the link.
(Other definitions for kinship that I've seen before include "A sharing of characteristics" , "Close connection marked by interests, similarity in nature, or character" , "Blood relationship" , "Blood tie" , "Having relations with" .)