Wines turn out to be a source of strength (5)
I believe the answer is:
sinew
'a source of strength' is the definition.
(I've seen this in another clue)
'wines turn out' is the wordplay.
'turn out' indicates an anagram.
'wines' is an anagram of 'SINEW'.
'to be' acts as a link.
(Other definitions for sinew that I've seen before include "Tough connective tissue" , "Cord of tissue connecting muscle to bone" , "source of physical strength" , "Inelastic tissue" , "Muscular strength" .)