Bound to allow one in to grumble (5)
I believe the answer is:
bleat
'to grumble' is the definition.
(both can mean to complain)
'bound to allow one in' is the wordplay.
'bound to' becomes 'bt' (this might be a standard abbreviation which I don't know about).
'allow' becomes 'le' (I've seen this in other clues).
'one' becomes 'a' (a thing is one thing).
'in' indicates putting letters inside.
'le'+'a'='lea'
'bt' enclosing 'lea' is 'BLEAT'.
Can you help me to learn more?
(Other definitions for bleat that I've seen before include "Cry plaintively" , "Complaint from Cotswold" , "Sound made by goat" , "Sheep's weak cry" , "Sound of sheep or goat" .)