Bit of a clever yarn, it's true (4)
I believe the answer is:
very
'true' is the definition.
(I've seen this in other clues)
'bit of a clever yarn it's' is the wordplay.
'bit of' indicates a hidden word.
'VERY' is hidden within 'clever yarn its'.
(Other definitions for very that I've seen before include "jolly British!" , "light, perhaps" , "Ever so" , "Truly" , "type of light" .)