Guess I am stopping baby's development; not good (10)
I believe the answer is:
estimation
'guess' is the definition.
(I've seen this before)
'i am stopping baby's development not good' is the wordplay.
'i am' becomes 'im'.
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'baby's development' becomes 'gestation'.
'not' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gestation' with 'g' taken out is 'estation'.
'im' put inside 'estation' is 'ESTIMATION'.
(Other definitions for estimation that I've seen before include "Judgment" , "Opinion" , "Guess" , "Reckoning of value" , "View" .)