Crewmen given fish: this seals deal (9)
I believe the answer is:
handshake
'this seals deal' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'crewmen given fish' is the wordplay.
'crewmen' becomes 'hands' (I've seen this in another clue).
'given' is a charade indicator (letters next to each other).
'fish' becomes 'hake' (hake is a kind of fish).
'hands'+'hake'='HANDSHAKE'
Can you help me to learn more?
(Other definitions for handshake that I've seen before include "Gesture of greeting or of agreeing a deal" , "acknowledgment" , "Physical greeting" , "Form of greeting" , "Conventional greeting of manual kind" .)