Yearn to seize booty I’m regularly plundering (7)
I believe the answer is:
looting
'plundering' is the definition.
(looting is a kind of plundering)
'yearn to seize booty i'm regularly' is the wordplay.
'yearn' becomes 'long-' (synonyms).
'to seize' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'booty'+'im'='bootyim'
The alternate letters of 'bootyim' are 'oti'.
'long' placed around 'oti' is 'LOOTING'.
(Other definitions for looting that I've seen before include "Wartime stealing" , "Sacking" , "Wartime theft" , "Stealing in times of unrest" , "Stealing during a disturbance" .)