Ally in uniform caught man on board (5)
I believe the answer is:
unite
'ally' is the definition.
(I've seen this before)
'uniform caught man on board' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'caught' indicates a 'sounds like' (homophone) clue (heard).
'man on board' becomes 'knight' (chess piece - man can mean a chess piece).
'knight' is a homophone of 'nite'.
'u'+'nite'='UNITE'
'in' acts as a link.
(Other definitions for unite that I've seen before include "Join together, become allies" , "Club together" , "Pull together" , "Bring together, become one" , "Gather together as one" .)