Something to swallow, if made up can be an incentive (6)
I believe the answer is:
fillip
'an incentive' is the definition.
(thesaurus)
'something to swallow if made up' is the wordplay.
'something to swallow' becomes 'pill' (I can't explain this - if you can you should believe this answer much more).
'made up' says the letters should be written backwards (in a down clue, letters go up).
'pill'+'if'='pillif'
'pillif' in reverse letter order is 'FILLIP'.
'can be' is the link.
Can you help me to learn more?
(Other definitions for fillip that I've seen before include "Something adding stimulation" , "Encouraging boost" , "If pill could be used as a stimulant" , "Incitement" , "lift" .)