A naturalist primarily twitches for larks (6)
I believe the answer is:
antics
'larks' is the definition.
'a naturalist primarily twitches' is the wordplay.
'primarily' says to take the initial letters.
'twitches' becomes 'tics' (tic is a kind of twitch).
The initial letter of 'naturalist' is 'n'.
'a'+'n'+'tics'='ANTICS'
'for' is the link.
(Other definitions for antics that I've seen before include "Funny movements and foolish behaviour" , "Goings-on, absurd behaviour" , "Absurd or foolish movements intended to amuse" , "Amusing behaviour" , "Movements intended to cause amusement" .)