Act as a trade union for sailors (4)
I believe the answer is:
turn
'act' is the definition.
(a routine or performance)
'a trade union for sailors' is the wordplay.
'a trade union' becomes 'TU' (abbreviation for trade union).
'for' is a charade indicator (letters next to each other).
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'tu'+'rn'='TURN'
'as' acts as a link.
(Other definitions for turn that I've seen before include "Refuse" , "fit" , "(Of tide) change from flood to ebb" , "revolutionary act" , "Move around an axis, or change" .)