Test canvas for use in bad weather (7)
I believe the answer is:
trysail
'for use in bad weather' is the definition.
(sail used during bad weather)
'test canvas' is the wordplay.
'test' becomes 'try' (synonyms).
'canvas' becomes 'sail' (synonyms).
'try'+'sail'='TRYSAIL'
(Other definitions for trysail that I've seen before include "Small piece of rigging" , "piece of canvas" , "Small strong canvas used in stormy weather" .)