Marrow in haggis? Turnips apparently (4)
I believe the answer is:
gist
'marrow' is the definition.
('gist' can be a synonym of 'marrow')
'in haggis? turnips apparently' is the wordplay.
'in' says the answer is hidden in the clue.
'GIST' can be found hidden inside 'haggis turnips apparently'.
(Other definitions for gist that I've seen before include "The central point of anything" , "Basic meaning" , "Main point of the matter" , "The substance of a text" , "General meaning or main point" .)