Capital fellow taking on jerk like Lothario? (8)
I believe the answer is:
romantic
'like lothario?' is the definition.
'capital fellow taking on jerk' is the wordplay.
'capital fellow' becomes 'roman'.
'taking on' is a charade indicator (letters next to each other).
'jerk' becomes 'tic' (I've seen this before).
'roman'+'tic'='ROMANTIC'
(Other definitions for romantic that I've seen before include "as Darnay perhaps" , "Amorous, showing loving feeling" , "Relating to love" , "Soulful or amourous idealist" , "Literary period in early 19th century" .)