Weak force on track (5)
I believe the answer is:
frail
'weak' is the definition.
(I know that weak can be written as frail)
'force on track' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'track' becomes 'rail' (synonyms).
'f'+'rail'='FRAIL'
(Other definitions for frail that I've seen before include "Feeble - basket" , "Slight" , "Basket made of rushes" , "Weak and infirm" , "Puny" .)