Turn 50 with day at space fair (6)
I believe the answer is:
golden
'fair' is the definition.
(thesaurus)
'turn 50 with day at space' is the wordplay.
'turn 50 with' becomes 'gol' (I can't justify this - if you can you should give a lot more credence to this answer).
'day' becomes 'd' (abbreviation for day).
'at' says to put letters next to each other.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'gol'+'d'+'en'='GOLDEN'
Can you help me to learn more?
(Other definitions for golden that I've seen before include "Gilded, pale yellow" , "- - wedding (50th)" , "With sixteen across Apple variety" , "Excellent" , "Autumnal flower" .)