Being physically fit, filled with energy in the meantime (7)
I believe the answer is:
interim
'the meantime' is the definition.
(I've seen this before)
'being physically fit filled with energy' is the wordplay.
'being physically fit' becomes 'intrim' (I've seen this in another clue).
'filled with' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'intrim' going around 'e' is 'INTERIM'.
'in' is the link.
(Other definitions for interim that I've seen before include ", I'm put in grave situation initially" , "Stopgap" , "not there for ever" , "An intervening period of time" , "Meanwhile" .)