Physical strength is back, regenerated (5)
I believe the answer is:
sinew
'physical strength' is the definition.
(I've seen this before)
'is back regenerated' is the wordplay.
'back' is a reversal indicator.
'regenerated' becomes 'new'.
'is' written backwards gives 'si'.
'si'+'new'='SINEW'
(Other definitions for sinew that I've seen before include "Tough material" , "Inelastic tissue" , "Strength; tendon" , "Tough connective tissue" , "It indicates strength" .)