Small wines become 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 become' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'become' is an anagram indicator.
'wines' with letters rearranged gives 'inews'.
's'+'inews'='SINEWS'
(Other definitions for sinews that I've seen before include "Tendons, ligaments" , "Power" , "Strength" , "Tough tissues joining muscles to bone" , "They connect muscle to bone" .)