Unique, extremely open, lovely! (4)
I believe the answer is:
only
'unique' is the definition.
(synonyms)
'extremely open lovely' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'lovely' becomes 'ly' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'open' with its centre taken out is 'on'.
'on'+'ly'='ONLY'
Can you help me to learn more?
(Other definitions for only that I've seen before include "And nothing more; sole" , ". . . . the Lonely (sung by Roy Orbison)" , "Except that; single" , "Siblingless (child)" , "And no one else" .)