Barely using energy, wife shows physical strength (5)
I believe the answer is:
sinew
'physical strength' is the definition.
(I've seen this before)
'barely using energy wife' is the wordplay.
'barely' says to take the centre (the word is bare of its external letters).
'energy' becomes 'e' (physics symbol as in E=mc2).
'wife' becomes 'w'.
The middle letters of 'using' are 'sin'.
'sin'+'e'+'w'='SINEW'
'shows' is the link.
(Other definitions for sinew that I've seen before include "Gristle" , "fibre" , "Inelastic tissue" , "Tendon, for example" , "power source" .)