Spar to support a sail (4)
I believe the answer is:
mast
'spar to support a sail' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mast that I've seen before include "Acorns, etc., fed to pigs" , "Supporting pole, for radio aerial, perhaps" , "Pole on ship" , "Long upright pole that supports sails" , "Pole aboard; fruit for pigs" .)