Forage around, conserving energy for sure (4)
I believe the answer is:
yeah
'sure' is the definition.
(thesaurus)
'forage around conserving energy' is the wordplay.
'forage' becomes 'hay' (I've seen this before).
'around' says the letters should be written backwards.
'conserving' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'hay' reversed gives 'yah'.
'yah' enclosing 'e' is 'YEAH'.
'for' is the link.
(Other definitions for yeah that I've seen before include "casual agreement" , "Indeed" , "quite casually" , "positive response?" , "OK" .)