Measure mercy, with no hesitation (5)
I believe the answer is:
quart
'measure' is the definition.
(quart is a kind of measure)
'mercy with no hesitation' is the wordplay.
'mercy' becomes 'quarter' (I've seen this in another clue).
'with no' indicates named letters should be taken away.
'hesitation' becomes 'er' (sound made while hesitating - um, er).
'quarter' with 'er' taken out is 'QUART'.
(Other definitions for quart that I've seen before include "water may be measured in this" , "A couple of pints" , "Liquid measure" , "The capacity" , "Two imperial pints" .)