Start to plead for an end to pain (5)
I believe the answer is:
begin
'start' is the definition.
(I know that start can be written as begin)
'plead for an end to pain' is the wordplay.
'plead' becomes 'beg' (begging is a kind of pleading).
'for' means one lot of letters go next to another.
'an' becomes 'i' (Roman numeral for one).
'end to' says to take the final letters.
The last letter of 'pain' is 'n'.
'beg'+'i'+'n'='BEGIN'
'to' acts as a link.
(Other definitions for begin that I've seen before include "Launch" , "Get cracking" , "Found" , "Make a start" , "Partly open" .)