Habitually fit some plastic to retain temperature (8)
I believe the answer is:
ofttimes
'habitually' is the definition.
(thesaurus)
'fit some plastic to retain temperature' is the wordplay.
'plastic' indicates anagramming the letters (plastic can mean easily moulded).
'to retain' means one lot of letters goes inside another.
'temperature' becomes 't' (abbreviation).
'fit'+'some'='fitsome'
'fitsome' is an anagram of 'oftimes'.
'oftimes' going around 't' is 'OFTTIMES'.
(Another definition for ofttimes that I've seen is " Frequently, at short intervals (arch/poetic)".)