Start missing in request (3)
I believe the answer is:
beg
'request' is the definition.
(I know that beg is a type of plead)
'start missing in' is the wordplay.
'start' becomes 'begin' (synonyms).
'missing' suggests deleting specific letters.
'begin' with 'in' taken away is 'BEG'.
(Other definitions for beg that I've seen before include "Implore or cadge" , "Plead as for help" , "Get by any means - . . ., borrow or steal" , "Ask for money, scrounge" , "Plead, implore" .)