A diver abandoned by bachelor in Grand at one (8)
I believe the answer is:
agreeing
'at one' is the definition.
(I've seen this before)
'a diver abandoned by bachelor in grand' is the wordplay.
'diver' becomes 'grebe' (diving bird).
'abandoned by' suggests deleting specific letters.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'grand' becomes 'g' (G is short for grand as in a thousand).
'grebe' with 'b' taken away is 'gree'.
'a'+'gree'+'in'+'g'='AGREEING'
(Other definitions for agreeing that I've seen before include "achieving harmony" , "Holding same opinion" , "Settling on" , "Concurring" , "OK" .)