He's after Tom with a leather strap. Big trouble! (11)
I believe the answer is:
catastrophe
'trouble' is the definition.
(catastrophe is a kind of trouble)
'he's after tom with a leather' is the wordplay.
'after' is a charade indicator (letters next to each other).
'tom' becomes 'cat' (a tom is a male cat).
'with' means one lot of letters go next to another.
'leather' becomes 'strop' (I've seen this before).
'cat'+'a'+'strop'='catastrop'
'he' after 'catastrop' is 'CATASTROPHE'.
'strap big' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for catastrophe that I've seen before include "Tragedy" , "Destructive event" , "Great disaster" , "Utter disaster" , "disastrous event?" .)