Enter lad disguised as a stag? (8)
I believe the answer is:
antlered
'as a stag?' is the definition.
'enter lad disguised' is the wordplay.
'disguised' indicates anagramming the letters.
'enter'+'lad'='enterlad'
'enterlad' with letters rearranged gives 'ANTLERED'.
(Other definitions for antlered that I've seen before include "(Of deer) having horns" , "Horned like deer" , "Dancer, for example, so" , "with horny growth" .)