Employ completely and beneficially (8)
I believe the answer is:
usefully
'beneficially' is the definition.
(I've seen this in another clue)
'employ completely' is the wordplay.
'employ' becomes 'use' (synonyms).
'completely' becomes 'fully' (in full).
'use'+'fully'='USEFULLY'
'and' acts as a link.
(Other definitions for usefully that I've seen before include "in handy manner" , "in a way that's beneficial" , "quite profitably" , "in helpful manner" , "To produce a benefit" .)