Get on unusually nicely at first in test (5)
I believe the answer is:
mount
'get on' is the definition.
('mount' can be a synonym of 'get on')
'unusually nicely at first in test' is the wordplay.
'at first' suggests taking the first letters.
'in' indicates putting letters inside.
'test' becomes 'mot'.
The initial letters of 'unusually nicely' is 'un'.
'un' placed inside 'mot' is 'MOUNT'.
(Other definitions for mount that I've seen before include "Climb - Everest, for example" , "Clamber on" , "seat" , "stage" , "Ascend; horse" .)