Latest thing I developed includes popular final surprise (5,2,3,4)
I believe the answer is:
sting in the tail
'final surprise' is the definition.
(phrase meaning an unexpected or unpleasant ending)
'latest thing i developed includes popular' is the wordplay.
'developed' indicates anagramming the letters.
'includes' means one lot of letters goes inside another.
'popular' becomes 'in' ('in' can mean trendy).
'latest'+'thing'+'i'='latestthingi'
'latestthingi' with letters rearranged gives 'stginthetail'.
'stginthetail' placed around 'in' is 'STING IN THE TAIL'.
(Other definitions for sting in the tail that I've seen before include "Unexpected final barb" , "Unpleasant end" , "Unpleasant ending" , "Unpleasant surprise" .)