The chaplain had thus been lying to a fellow (4)
I believe the answer is:
lain
'fellow' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally define each other.
'the chaplain had thus been lying' is the wordplay.
'LAIN' is hidden within the letters.
However, I am uncertain how the hidden word is indicated.
'to a' is the link.
Can you help me to learn more?
(Other definitions for lain that I've seen before include "Having been put flat (on the ground)" , "Rested; nail (anag)" , "Past participle of lie" , "Set down horizontally" , "(Had) remained" .)