Force hard when cold (6)
I believe the answer is:
frigid
'cold' is the definition.
(similar in meaning)
'force hard' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'hard' becomes 'rigid' (I've seen this before).
'f'+'rigid'='FRIGID'
'when' is the link.
(Other definitions for frigid that I've seen before include "Lacking affection or warmth" , "Passionless, unresponsive" , "Sexually unresponsive" , "Very cold; formal in behaviour" , "Glacial" .)