Force to modify flag (6)
I believe the answer is:
falter
'flag' is the definition.
(thesaurus)
'force to modify' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'to modify' becomes 'alter' (I've seen this before).
'f'+'alter'='FALTER'
(Other definitions for falter that I've seen before include "Hesitate, begin to give way" , "Hesitate, be uncertain" , "flag" , "Lose momentum" , "Be hesitant" .)