Farm animal’s light weight, not good (3)
I believe the answer is:
ram
'farm animal's' is the definition.
(male sheep)
'light weight not good' is the wordplay.
'light weight' becomes 'gram' (I've seen this in another clue).
'not' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gram' with 'g' taken away is 'RAM'.
(Other definitions for ram that I've seen before include "needed for PC" , "Tamp" , "'Computer memory, initially (3)'" , "Aries is the Sign of the ..." , "Drive hard" .)