Respond with exaggeration on tucking into ham (9)
I believe the answer is:
overreact
'respond with exaggeration' is the definition.
(to react in an exaggerated way)
'on tucking into ham' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'tucking into' is an insertion indicator.
'ham' becomes 'overact' (ham can mean to overact).
're' placed inside 'overact' is 'OVERREACT'.
(Other definitions for overreact that I've seen before include "Respond excessively" , "Respond too much" , "Respond too strongly" , "Show too much emotion" , "Get too excited" .)