Open using force, standing underneath (5)
I believe the answer is:
frank
'open' is the definition.
(synonyms)
'force standing underneath' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'standing' becomes 'rank' (as in social standing or rank).
'underneath' says to put letters next to each other (in a down clue, some letters go under others).
'f'+'rank'='FRANK'
'using' is the link.
(Other definitions for frank that I've seen before include "stamp" , "Blunt; honest" , "Old German" , "Candid, honest" , "Anne ---, diarist" .)