Spike Norse god’s nectar at first (5)
I believe the answer is:
thorn
'spike' is the definition.
(I've seen this before)
'norse god's nectar at first' is the wordplay.
'norse god' becomes 'thor' (Thor is an example).
'at first' indicates taking the first letters.
The initial letter of 'nectar' is 'n'.
'thor'+'n'='THORN'
(Other definitions for thorn that I've seen before include "Sharp point on a shrub" , "It causes irritation and annoyance" , "North (anag) -- point" , "Spine - prickle - Old English runic letter" , "Prickle on a plant" .)