Swallowing tablet, recover in truth (6)
I believe the answer is:
really
'in truth' is the definition.
(I know that in truth can be written as really)
'swallowing tablet recover' is the wordplay.
'swallowing' means one lot of letters goes inside another.
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'recover' becomes 'rally' (rallying is a kind of recovering).
'e' going into 'rally' is 'REALLY'.
(Other definitions for really that I've seen before include "surprised expression" , "without a doubt" , "Well I never!" , "In genuine way" , "are you sure?" .)