One taking a bow when first in some vivas in biology (4,4)

'one taking a bow' is the definition.
(instrument played with a bow)

'when first in some vivas in biology' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'first in' suggests taking the first letters.
'in' indicates putting letters inside.
'biology' becomes 'biol' (abbreviation).
The first letters of 'some vivas' is 'sv'.
'as'+'sv'='assv'
'assv' inserted into 'biol' is 'BASS VIOL'.