Boring commercial for wood spirit (5)
I believe the answer is:
dryad
'wood spirit' is the definition.
(I've seen this before)
'boring commercial' is the wordplay.
'boring' becomes 'dry' (dry can mean dull or boring).
'commercial' becomes 'ad' (abbreviation for advertisement).
'dry'+'ad'='DRYAD'
'for' acts as a link.
(Other definitions for dryad that I've seen before include "one stuck in a tree" , "minor divinity" , "Nymph of the woods" , "butterfly" , "Nymph of trees" .)