Outstanding alien sitting in middle of street (7)
I believe the answer is:
salient
'outstanding' is the definition.
(synonyms)
'alien sitting in middle of street' is the wordplay.
'sitting in' is an insertion indicator.
'middle of street' becomes 'st' (I can't justify this - if you can you should give a lot more credence to this answer).
'alien' put into 'st' is 'SALIENT'.
Can you help me to learn more?
(Other definitions for salient that I've seen before include "Prominent - in tales (anag)" , "Entails being most noticeable" , "Noteworthy" , "Entails (anag) - most important" , "Leaping (heraldry)" .)