Start to observe twitching of the ear (4)
I believe the answer is:
otic
'of the ear' is the definition.
(otic means relating to the ear)
'start to observe twitching' is the wordplay.
'start to' says to take the initial letters.
'twitching' becomes 'tic' (tic is a kind of twitching).
The first letter of 'observe' is 'o'.
'o'+'tic'='OTIC'
(Other definitions for otic that I've seen before include "in relation to the listener" , "organ" , "of lugs" , "Listener's" , "Relating to the ear" .)