Concern about sailor: source of trouble in nightclub (7)
I believe the answer is:
cabaret
'nightclub' is the definition.
(synonyms)
'concern about sailor source of trouble' is the wordplay.
'concern' becomes 'care' (synonyms).
'about' is an insertion indicator.
'sailor' becomes 'ab' (abbreviation for able seaman).
'source of' indicates taking the first letters.
The initial letter of 'trouble' is 't'.
'care' placed around 'ab' is 'cabare'.
'cabare'+'t'='CABARET'
'in' acts as a link.
(Other definitions for cabaret that I've seen before include "Variety entertainment" , "Entertainment at restaurant" , "Musical - nightclub entertainment" , "Floor-show as in nightclub" , "Entertainment at night club" .)