Expresses what guttersnipe is concealing (6)
I believe the answer is:
Here is my best explanation:
'expresses' is the definition.
('utter' can be a synonym of 'express')
'what guttersnipe is concealing' is the subsidiary indication.
'is concealing' indicates a hidden word.
'utters' is hidden within 'what guttersnipe'.
(Other definitions for utters that I've seen before include "States, articulate", "Expresses audibly", "Says", "Speaks, articulates", "Articulates, expresses", "Speaks, voices", "Speaks, says" and "Gives vocal expression to".)