Joins up but listens carelessly (7)
I believe the answer is:
enlists
'joins up' is the definition.
(I've seen this before)
'listens carelessly' is the wordplay.
'carelessly' is an anagram indicator.
'listens' with letters rearranged gives 'ENLISTS'.
'but' is the link.
(Other definitions for enlists that I've seen before include "Signs up" , "Registers" , "Recruits" , "gets involved" , "volunteers" .)