Boring commercial for spirit (5)
I believe the answer is:
dryad
'for spirit' is the definition.
(type of spirit in Greek myth)
'boring commercial' is the wordplay.
'boring' becomes 'dry' (dry can mean dull or boring).
'commercial' becomes 'ad' (abbreviation for advertisement).
'dry'+'ad'='DRYAD'
(Other definitions for dryad that I've seen before include "Wood-nymph" , "Beautiful young woman" , "Woodland nymph" , "Tree nymph" , "Spirit in the wood" .)