Thrill — not half — over start of bass beat (5)
I believe the answer is:
throb
'beat' is the definition.
(throb is a kind of beat)
'thrill not half over start of bass' is the wordplay.
'not half' suggests halving the letters.
'start of' says to take the initial letters (I've seen 'start' mean this).
'bass' becomes 'b' (musical abbreviation).
The first letter of 'over' is 'o'.
'thrill' halved is 'thr'.
'thr'+'o'+'b'='THROB'
(Other definitions for throb that I've seen before include "Pound" , "Strong rhythmic beat" , "Beat like heart" , "Martin -; Kingsley -" , "Have strong or irregular heartbeat" .)