Ultimately, love rat possibly caustic (7)
I believe the answer is:
erodent
'caustic' is the definition.
(I've seen this before)
'ultimately love rat possibly' is the wordplay.
'ultimately' says to take the final letters.
'rat possibly' becomes 'rodent' (rat is a kind of rodent).
The final letter of 'love' is 'e'.
'e'+'rodent'='ERODENT'
(Other definitions for erodent that I've seen before include "eating away" , "Caustic" , "Corrosive" , "wearing away" .)