Having bad effect on mother, a drink, little good (8)
I believe the answer is:
damaging
'having bad effect on' is the definition.
'mother a drink little good' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'drink' becomes 'gin' (gin is a kind of drink**).
'little' suggests taking the first letters.
The initial letter of 'good' is 'g'.
'dam'+'a'+'gin'+'g'='DAMAGING'
(Other definitions for damaging that I've seen before include "injurious" , "Causing harm or injury" , "Hurting" .)