Compulsion to work and press together when speaking (4)
I believe the answer is:
need
'compulsion' is the definition.
(a compulsion is a need to do something)
'to work and press together when speaking' is the wordplay.
'to work and press together' becomes 'knead' (as in kneading dough).
'when speaking' indicates a 'sounds like' (homophone) clue.
'knead' is a homophone of 'NEED'.
(Other definitions for need that I've seen before include "Require; want" , "Lack - demand" , "What's lacking" , "''A friend in .... is a friend indeed''" , "state of distress" .)