Responds excessively when engineer gets stuck into ham (10)

I believe the answer is:

overreacts

'responds excessively' is the definition.

'engineer gets stuck into ham' is the wordplay.
'engineer' becomes 're' (Royal Engineer).
'gets stuck into' means one lot of letters goes inside another.
'ham' becomes 'overacts'.
're' going into 'overacts' is 'OVERREACTS'.

'when' is the link.

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