A loud, boring guest occasionally gives inspiration (8)
I believe the answer is:
afflatus
'inspiration' is the definition.
(afflatus is a kind of inspiration)
'a loud boring guest occasionally' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'boring' becomes 'flat' (flat can mean dull or lifeless).
'occasionally' indicates alternate letters.
The alternating letters of 'guest' are 'us'.
'a'+'f'+'flat'+'us'='AFFLATUS'
'gives' is the link.
(Other definitions for afflatus that I've seen before include "inspiration" , "Poet's creative impulse" .)