Reserve shown in Punic engagement (3)
I believe the answer is:
ice
'reserve' is the definition.
(both can mean aloofness)
'in punic engagement' is the wordplay.
'in' says the answer is hidden in the clue.
'ICE' is hidden within 'punic engagement'.
'shown' is the link.
(Other definitions for ice that I've seen before include "Diamonds, colloquially" , "Top cake" , "engine" , "Frozen liquid" , "Glaciate" .)