Settle on a clue (6)
I believe the answer is:
alight
'settle' is the definition.
(I've seen this before)
'on a clue' is the wordplay.
'on' is a charade indicator (letters next to each other).
'clue' becomes 'light'.
'a' next to 'light' is 'ALIGHT'.
(Other definitions for alight that I've seen before include "Get down from, say bus" , "Descend from vehicle" , "Descend, from a bus or train say" , "staying in?" , "(Of bird) settle" .)