Slaver's keener to acquire second of plantations (7)
I believe the answer is:
slobber
'slaver's' is the definition.
(synonyms)
'keener to acquire second of plantations' is the wordplay.
'keener' becomes 'sobber' (keen can mean to cry or lament).
'to acquire' is an insertion indicator.
'second of plantations' becomes 'l' (2nd letter of 'plantations').
'sobber' enclosing 'l' is 'SLOBBER'.
(Other definitions for slobber that I've seen before include "Slaver" , "Drip saliva" , "Behave with excessive affection, dribble" , "Salivate excessively" , "Drool disgustingly" .)