Pretends to be a radio operator in a ship (5)
I believe the answer is:
shams
'pretends' is the definition.
(I know this)
'a radio operator in a ship' is the wordplay.
'a radio operator' becomes 'ham' (I've seen this in another clue).
'in' is an insertion indicator.
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ham' going into 'ss' is 'SHAMS'.
'to be' is the link.
(Other definitions for shams that I've seen before include "Fakers, pseuds" , "Pretences" .)