Gleam when he's in trouble (5)
I believe the answer is:
shine
'gleam' is the definition.
('shine' can be a synonym of 'gleam')
'he's in trouble' is the wordplay.
'trouble' indicates an anagram.
'hes'+'in'='hesin'
'hesin' is an anagram of 'SHINE'.
'when' acts as a link.
(Other definitions for shine that I've seen before include "Beam" , "Give out or reflect light" , "Glimmer" , "Glow - film about pianist" , "Give out light; polish" .)