Dad's climbing a lot of trees to find what lies below bark (7)

'what lies below bark' is the definition.
(layer of wood below bark)

'dad's climbing a lot of trees' is the wordplay.
'dad's' becomes 'pa's' ('pa' can be a synonym of 'dad').
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'a lot of trees' becomes 'wood'.
'pas' in reverse letter order is 'sap'.
'sap'+'wood'='SAPWOOD'

'to find' is the link.