N Armstrong, I fancy, is an astronomical phenomenon (7,4)
I believe the answer is:
morning star
'an astronomical phenomenon' is the definition.
(I know that morning star is a type of major planet)
'n armstrong i fancy' is the wordplay.
'fancy' indicates an anagram.
'n'+'armstrong'+'i'='narmstrongi'
'narmstrongi' is an anagram of 'MORNING-STAR'.
'is' acts as a link.
(Other definitions for morning star that I've seen before include "Paper" , "one prominent on breakfast TV?" , "one up bright and early" , "Venus before sunrise" , "smaller rival?" .)