Horseman it's said put on uniform to join forces (5)
I believe the answer is:
unite
'join forces' is the definition.
(thesaurus)
'horseman it's said put on uniform' is the wordplay.
'horseman' becomes 'knight' (I've seen this before).
'it's said' shows a homophone (sound like).
'put on' says to put letters next to each other.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'knight' sounds like 'nite'.
'nite' put after 'u' is 'UNITE'.
'to' acts as a link.
(Other definitions for unite that I've seen before include "Come together as allies or comrades" , "Pull together" , "fuse" , "Join together, merge" , "Make one" .)