Responds excessively when engineer gets stuck into ham (10)
I believe the answer is:
Here is my best explanation:
'responds excessively' is the definition.
'engineer gets stuck into ham' is the subsidiary indication.
'engineer' becomes 're' (Royal Engineer).
'gets stuck into' means one lot of letters goes inside another.
'ham' becomes 'overacts'.
're' going inside 'overacts' is 'overreacts'.
'when' is the link.