Slam, locking, with the key inside and they bleat (5)
I believe the answer is:
lambs
'and they bleat' is the definition.
The definition suggests an adverb but the answer is not.
'slam locking with the key' is the wordplay.
I cannot quite see how this works, but
'key' could be 'b' (eg B major, musical key) and 'b' is present in the answer.
an anagram of 'slam' is 'lams' which is within the left over letters.
This accounts for all the letters.
This explanation may well be incorrect...
'inside' is the link.
Can you help me to learn more?
(Other definitions for lambs that I've seen before include "See 17" , "Young sheep" .)