Small wines generate sources of strength (6)
I believe the answer is:
sinews
'sources of strength' is the definition.
(I've seen this in another clue)
'small wines generate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'generate' indicates an anagram.
'wines' with letters rearranged gives 'inews'.
's'+'inews'='SINEWS'
(Other definitions for sinews that I've seen before include "Power" , "They connect muscle to bone" , "Tissues connecting muscles to bones" , "Tough tissues joining muscles to bone" , "Tissue connecting muscles to bones" .)