Speak unclearly: start to moan like Uriah Heep? (6)
I believe the answer is:
mumble
'speak unclearly' is the definition.
(I've seen this before)
'start to moan like uriah heep?' is the wordplay.
'start to' indicates taking the first letters.
'like uriah heep?' becomes 'umble' (I've seen this in another clue).
The first letter of 'moan' is 'm'.
'm'+'umble'='MUMBLE'
(Other definitions for mumble that I've seen before include "indistinct utterance" , "Speak inarticulately" , "Mutter" , "Speak unclearly" , "Speak indistinctly, fail to articulate" .)