Go out and finally purchase two books (3)
I believe the answer is:
ebb
'go out' is the definition.
(as in the tide going out)
'finally purchase two books' is the wordplay.
'two books' can mean two substitutions for 'book'.
'finally' says to take the final letters.
'book' becomes 'b' (abbreviation for book).
'book' becomes 'b' (abbreviation for book).
The final letter of 'purchase' is 'e'.
'e'+'b'+'b'='EBB'
'and' is the link.
(Other definitions for ebb that I've seen before include "(Of tidewater) recede" , "Receding movement, as of a tide" , "Initially Mrs Browning's" , "Fall back" , "(Water) recede" .)