Struck with top of southpaw’s boxing glove (7)
I believe the answer is:
Here is my best explanation:
'struck with' is the definition.
'top of southpaw's boxing glove' is the wordplay.
'top of' suggests taking the first letters.
'boxing glove' becomes 'mitten' (I've seen this before).
The initial letter of 'southpaw' is 's'.
(Other definitions for smitten that I've seen before include "Infatuated with mittens", "Strongly affected (by something)", "Much taken by", "Struck (by love?)", "Head over heels in love", "Severely affected (by something)", "Besotted, enamoured", "Hit (by remorse, love)", "Very much in love" and "Affected or struck by, disease or desire".)