Gestures for gin cocktail aboard ship (5)
I believe the answer is:
signs
'gestures' is the definition.
(I know this)
'gin cocktail aboard ship' is the wordplay.
'cocktail' is an anagram indicator.
'aboard' indicates putting letters inside.
'ship' becomes 'ss' (prefix in ship names eg SS Great Britain).
'gin' is an anagram of 'ign'.
'ign' put inside 'ss' is 'SIGNS'.
'for' acts as a link.
(Other definitions for signs that I've seen before include "Autographs" , "Marks" , "Pens name" , "Indications" , "Mathematician's symbols" .)