Good to enter royal house? It can be a headache (8)
I believe the answer is:
hangover
'it can be a headache' is the definition.
'good to enter royal house?' is the wordplay.
'good' becomes 'g' (abbreviation).
'to enter' indicates putting letters inside.
'royal house?' becomes 'hanover' (Hanover is an example).
'g' put within 'hanover' is 'HANGOVER'.
(Other definitions for hangover that I've seen before include "result of over-indulgence?" , "Sick feeling the morning after the night before" , "After-effects of a binge" , "Unpleasant aftermath of drinking binge" , "Consequence of too much booze the night before" .)