Doctor fuels you, say, to become productive (6)
I believe the answer is:
useful
'productive' is the definition.
(thesaurus)
'doctor fuels you say' is the wordplay.
'doctor' indicates anagramming the letters.
'say' shows a homophone (sound like).
'you' sounds like 'u'.
'fuels'+'u'='fuelsu'
'fuelsu' is an anagram of 'USEFUL'.
'to become' is the link.
(Other definitions for useful that I've seen before include "Handy, practical" , "Functional and worthwhile" , "Suitable for practical purpose" , "Handy, serviceable" , "Handy; very able" .)