Small tract exhibits a lead-lined layer (8)

'small tract' is the definition.
'chapbook' can be an answer for 'tract' (thesaurus). I am not certain of the 'small' bit.

'exhibits a lead-lined layer' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'lead' could be 'pb' (Pb is the chemical symbol for lead) and 'pb' is located in the answer.
The remaining letters 'chook' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?