Deserted the French force by junction (4)
I believe the answer is:
left
'deserted' is the definition.
(I've seen this in another clue)
'the french force by junction' is the wordplay.
'the french' becomes 'le' ('the' in French).
'force' becomes 'f' (abbreviation in physics e.g. f=ma).
'by' says to put letters next to each other.
'junction' becomes 't'.
'le'+'f'+'t'='LEFT'
(Other definitions for left that I've seen before include "Departed, went away" , "political grouping" , "Withdrew - Labour" , "Communist?" , "The sinister side" .)