The French abandoned uncle, sent to the gallows in uniform (9)
I believe the answer is:
unchanged
'uniform' is the definition.
(thesaurus)
'the french abandoned uncle sent to the gallows' is the wordplay.
'the french' becomes 'le' ('the' in French).
'abandoned' indicates named letters should be taken away.
'sent to the gallows' becomes 'hanged' (I've seen this in another clue).
'uncle' with 'le' removed is 'unc'.
'unc'+'hanged'='UNCHANGED'
'in' is the link.
(Other definitions for unchanged that I've seen before include "Not altered" , "Unaltered" , "Remaining in an original state" .)