Sufficiency of drink surrounding politician on loch (9)

'sufficiency' is the definition.
(ampleness is a kind of sufficiency)

'drink surrounding politician on loch' is the wordplay.
'drink' becomes 'ale' (ale is a kind of drink).
'surrounding' is an insertion indicator.
'politician' becomes 'mp' (Member of Parliament).
'on' says to put letters next to each other.
'loch' becomes 'ness' (I've seen this before).
'ale' placed around 'mp' is 'ample'.
'ample'+'ness'='AMPLENESS'

'of' acts as a link.