Heads off to get small sunglasses (6)
I believe the answer is:
shades
'sunglasses' is the definition.
(synonyms)
'heads off to get small' is the wordplay.
'off' indicates an anagram.
'to get' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'heads' with letters rearranged gives 'shade'.
'shade'+'s'='SHADES'
(Other definitions for shades that I've seen before include "Sunglasses, colloquially" , "Old ghosts or modern sunglasses" , "Cool sunglasses or archaic ghosts" , "Sunglasses (informal)" , "Poetic ghosts or American sunglasses" .)