At first believe old butcher's lamb tender (6)

'lamb tender' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'at first believe old butcher's' is the wordplay.
'at' says to put letters next to each other.
'first believe' becomes 'b' (pst letter of 'believe').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'butcher's' becomes 'peep' (I can't justify this - if you can you should give a lot more credence to this answer).
'b' put next to 'o' is 'bo'.
'bo'+'peep'='BOPEEP'

Can you help me to learn more?