With no top on, insists on lifting weight (6)
I believe the answer is:
stress
'weight' is the definition.
(thesaurus)
'with no top on insists on lifting' is the wordplay.
'with no top on' means to remove the first letter (the first or top letter is removed).
'insists' becomes 'asserts' (asserting is a kind of insisting).
'on lifting' says the letters should be written in reverse (in a down clue, letters go up).
'asserts' with its initial letter taken off is 'sserts'.
'sserts' reversed gives 'STRESS'.
(Other definitions for stress that I've seen before include "Tension - accent" , "'Worry, pressure (6)'" , "Lay emphasis on - it's a strain" , "Ictus" , "Underline - tension" .)