Function requiring weight and source of strength (5)
I believe the answer is:
sinew
'source of strength' is the definition.
(I've seen this in another clue)
'function requiring weight' is the wordplay.
'function' becomes 'sine' (mathematical function).
'requiring' says to put letters next to each other.
'weight' becomes 'w' (short for weight).
'sine'+'w'='SINEW'
'and' acts as a link.
(Other definitions for sinew that I've seen before include "Tough connective tissue" , "Tendon, for example" , "It indicates strength" , "Gristle" , "Force" .)