He has to look out for trouble starting (5)
I believe the answer is:
peter
'he has to look out for trouble starting' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he has to look out' is the wordplay.
'he has' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'to look' becomes 'peer' (to peer at something is to look at it).
'out' means one lot of letters goes inside another.
't' put inside 'peer' is 'PETER'.
Can you help me to learn more?
(Other definitions for peter that I've seen before include "In whist, play" , "and 14 dn Latest actor to play Doctor Who" , "Play high card before low" , "Dwindle away to nothing" , "Some rob ..... to pay Paul" .)