Old friend acquires twitch relating to sight (7)
I believe the answer is:
optical
'relating to sight' is the definition.
(I've seen this before)
'old friend acquires twitch' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal can mean a friend).
'acquires' indicates putting letters inside.
'twitch' becomes 'tic' (tic is a kind of twitch).
'pal' placed around 'tic' is 'ptical'.
'o'+'ptical'='OPTICAL'
(Other definitions for optical that I've seen before include "Sort of illusion" , "Concerning sight" , "Using visible light" , "Relating to light/vision" , "Involving light" .)