Pleased, mostly, breaking out (6)
I believe the answer is:
asleep
'out' is the definition.
(I've seen this before)
'pleased mostly breaking' is the wordplay.
'mostly' means to remove the last letter.
'breaking' indicates an anagram.
'pleased' with its final letter taken away is 'please'.
'please' is an anagram of 'ASLEEP'.
(Other definitions for asleep that I've seen before include "Dormant, inactive" , "out like a light?" , "coiled up?" , "not paying attention" , "Dormant; inattentive" .)