Tonic following tablet I knocked back (6)

I believe the answer is:

fillip

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'tonic' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'following tablet i knocked back' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'tablet' becomes 'pill' (synonyms).
'knocked back' is a reversal indicator.
'pill'+'i'='pilli'
'pilli' written backwards gives 'illip'.
'f'+'illip'='FILLIP'

Can you help me to learn more?

(Other definitions for fillip that I've seen before include "shot in the arm" , "give encouragement" , "Stimulus, quick boost" , "Spur - incentive" , "Encouragement, stimulus" .)

I've seen this clue in The Times.

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