Act as a trade union for sailors (4)
I believe the answer is:
turn
'act' is the definition.
(theatre)
'a trade union for sailors' is the wordplay.
'a trade union' becomes 'TU' (abbreviation for trade union).
'for' says to put letters next to each other.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'tu'+'rn'='TURN'
'as' is the 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" .)