Knock back a tablet if you need a boost (6)
I believe the answer is:
fillip
'a boost' is the definition.
(I've seen this before)
'knock back a tablet if' is the wordplay.
'knock back' says the letters should be written in reverse.
'a tablet' becomes 'pill' ('pill' can be a synonym of 'tablet').
'pill'+'if'='pillif'
'pillif' in reverse letter order is 'FILLIP'.
'you need' acts as a link.
(Other definitions for fillip that I've seen before include "Spur - incentive" , "Something adding stimulation" , "If pill could be used as a stimulant" , "lift" , "Sharp stimulus or flick of a finger" .)