Recalled sullen nutritionist revealing what they might go through (6)
I believe the answer is:
tunnel
'they might go through' is the definition.
The definition suggests an adverb but the answer is not.
'recalled sullen nutritionist revealing' is the wordplay.
'recalled' is a reversal indicator (to recall can mean to bring something back).
'revealing' says the answer is hidden in the clue.
'lennut' can be found hidden inside 'sullen nutritionist'.
'lennut' reversed gives 'TUNNEL'.
'what' is the link.
Can you help me to learn more?
(Other definitions for tunnel that I've seen before include "Passage through or under something" , "See 11" , "Underground passage for road or railway" , "Passageway under something" , "See 6" .)