Thrash lightweight after first of two rounds (7)
I believe the answer is:
trounce
'thrash' is the definition.
(I've seen this before)
'lightweight after first of two rounds' is the wordplay.
'lightweight' becomes 'ounce' (I've seen this before).
'after' says to put letters next to each other.
'first of' suggests taking the first letters.
The initial letters of 'two rounds' is 'tr'.
'ounce' put after 'tr' is 'TROUNCE'.
(Other definitions for trounce that I've seen before include "Easily best" , "Beat hollow" , "Beat heavily" , "Beat soundly" , "National flower of South Africa" .)