Sober plug for spirit (5)
I believe the answer is:
dryad
'for spirit' is the definition.
(type of spirit in Greek myth)
'sober plug' is the wordplay.
'sober' becomes 'dry' (similar in meaning).
'plug' becomes 'ad' (as in plugging a product).
'dry'+'ad'='DRYAD'
(Other definitions for dryad that I've seen before include "Mythological wood nymph" , "butterfly" , "Beautiful young woman" , "Spirit in the wood" , "Nymph of trees" .)