Exactly the beginning of Sixties sounds (6)
I believe the answer is:
plumbs
'sounds' is the definition.
(I've seen this before)
'exactly the beginning of sixties' is the wordplay.
'exactly' becomes 'plumb' (I've seen this before).
'the beginning of' suggests taking the first letters.
The first letter of 'sixties' is 's'.
'plumb'+'s'='PLUMBS'
(Other definitions for plumbs that I've seen before include "Sounds" , "Reaches or experiences, say the depths" , "Gets to the bottom of" .)