The punishment I had was swift (5)
I believe the answer is:
rapid
'swift' is the definition.
(I've seen this in another clue)
'the punishment i had' is the wordplay.
'the punishment' becomes 'rap' (I've seen this in another clue).
'i had' becomes 'I'd' (contraction).
'rap'+'id'='RAPID'
'was' acts as a link.
(Other definitions for rapid that I've seen before include "Happening at great speed" , "Quick or swift" , "haste" , "Quickly accomplished" , "Fast, speedy" .)