Thor assembled on bobby's first beat (5)
I believe the answer is:
throb
'beat' is the definition.
(I know that throb is a type of beat)
'thor assembled on bobby's first' is the wordplay.
'assembled' indicates an anagram.
'on' is a charade indicator (letters next to each other).
'bobby's first' becomes 'b' (1st letter of 'bobby').
'thor' with letters rearranged gives 'thro'.
'thro'+'b'='THROB'
(Other definitions for throb that I've seen before include "Beat rhythmically" , "Broth (anag.)" , "Beat strongly" , "Pulse" , "Pulsate; regular beat" .)