Severe? It's in character, upon reflection (7)
I believe the answer is:
drastic
'severe?' is the definition.
(I've seen this before)
'it's in character upon reflection' is the wordplay.
'in' is an insertion indicator.
'character' becomes 'card' (I've seen this in other clues).
'upon reflection' shows that the letters should be reversed in order.
'its' put inside 'card' is 'citsard'.
'citsard' back-to-front is 'DRASTIC'.
(Other definitions for drastic that I've seen before include "Radical, brutal" , "Unsparing" , "Extremely severe" , "'Severe, extreme (7)'" , "Grave" .)