Take care of deterioration in muscle temperature (7)
I believe the answer is:
protect
'take care of' is the definition.
(thesaurus)
'deterioration in muscle temperature' is the wordplay.
'deterioration' becomes 'rot' (I've seen this before).
'in' is an insertion indicator.
'muscle' becomes 'pec' (I've seen this in other clues).
'temperature' becomes 't' (abbreviation).
'rot' inserted inside 'pec' is 'protec'.
'protec'+'t'='PROTECT'
(Other definitions for protect that I've seen before include "Keep from danger" , "Defend" , "Shield from danger" , "Keep safe" , "Take care of" .)