Unusual compass found under edges of seat (7)
I believe the answer is:
strange
'unusual' is the definition.
('strange' can be similar in meaning to 'unusual')
'compass found under edges of seat' is the wordplay.
'compass' becomes 'range' ('range' can be a synonym of 'compass').
'found under' means one lot of letters go next to another.
'edges of' suggests removing the centre.
'seat' with its middle taken out is 'st'.
'range' put after 'st' is 'STRANGE'.
(Other definitions for strange that I've seen before include "Funny" , "- - Fruit (song)" , "Odd - rum" , "Unknown or peculiar" , "Abnormal" .)