What's here in truth, I swear (4)
I believe the answer is:
this
'what's here' is the definition.
(I've seen this in another clue)
'in truth i swear' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THIS' can be found hidden inside 'truth i swear'.
(Other definitions for this that I've seen before include "Hits (anag.)" , "The thing under discussion" , "The thing here referred to" , "Demonstrative pronoun" , "Pronoun to point out nearby things" .)