Lament extinct bird in Scottish mountain (6)
I believe the answer is:
'lament' is the definition.
'extinct bird in scottish mountain' is the wordplay.
'extinct bird' becomes 'moa' (extinct flightless bird of New Zealand).
'in' indicates putting letters inside.
'scottish mountain' becomes 'ben' (Scottish term for a peak, eg Ben Nevis).
'moa' placed inside 'ben' is 'BEMOAN'.
(Other definitions for bemoan that I've seen before include "Complain about, lament" , "Regret, lament" , "Lament, grieve" , "Deplore, lament" .)