Developing ice in pint by mistake (9)
I believe the answer is:
incipient
'developing' is the definition.
(I've seen this in another clue)
'ice in pint by mistake' is the wordplay.
'by mistake' indicates an anagram.
'ice'+'in'+'pint'='iceinpint'
'iceinpint' with letters rearranged gives 'INCIPIENT'.
(Other definitions for incipient that I've seen before include "Beginning to develop" , "Starting to happen" , "Nascent" , "Developing" , "Beginning to exist, in an early stage" .)