Puzzle on coach (5)
I believe the answer is:
rebus
'puzzle' is the definition.
(type of visual puzzle)
'on coach' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'coach' becomes 'bus' (synonyms).
're'+'bus'='REBUS'
(Other definitions for rebus that I've seen before include "Puzzle where pictures represent syllables and words" , "Visual type of word puzzle" , "solver?" , "Pictorial wordplay" , "Pictorial pun - fictional detective" .)