Beer starts to affect lad excessively (3)
I believe the answer is:
ale
'beer' is the definition.
(I've seen this before)
'starts to affect lad excessively' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'affect lad excessively' is 'ALE'.
(Other definitions for ale that I've seen before include "Stout" , "brew" , "English beer, may be bitter" , "Booze" , "Beerlike beverage" .)