Undertone doubled the endless gloom (6)
I believe the answer is:
murmur
'undertone' is the definition.
(I've seen this in another clue)
'doubled the endless gloom' is the wordplay.
'doubled' indicates the doubling of some letters.
'the endless' means to remove the last letter.
'gloom' becomes 'murk' (I've seen this in another clue).
'murk' with its final letter removed is 'mur'.
'mur' doubled is 'MURMUR'.
(Other definitions for murmur that I've seen before include "Whisper; complain" , "Low continuous sound" , "Quietly spoken utterance; heart defect" , "Subdued complaint" , "Speak quietly" .)