Birds mostly peck, with one poking inside (5)
I believe the answer is:
kiwis
'birds' is the definition.
(kiwi is a kind of bird)
'mostly peck with one poking inside' is the wordplay.
'mostly' means to remove the last letter.
'peck' becomes 'kiss' (I've seen this before).
'with one' becomes 'wi' (I can't justify this - if you can you should believe this answer much more).
'poking inside' is an insertion indicator (I've seen this in other clues).
'kiss' with its final letter removed is 'kis'.
'kis' enclosing 'wi' is 'KIWIS'.
Can you help me to learn more?
(Other definitions for kiwis that I've seen before include "NZ birds or team" , "Flightless birds and fruits" , "Flightless birds of New Zealand" .)