Posh rival picked up in spaceship? (3)
I believe the answer is:
ufo
'spaceship?' is the definition.
(I have seen 'Potential spaceship ' mean 'ufo' so perhaps 'spaceship' could also mean 'ufo')
'posh rival picked up' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'rival' becomes 'of' (I can't justify this - if you can you should believe this answer much more).
'picked up' is a reversal indicator.
'of' in reverse letter order is 'fo'.
'u'+'fo'='UFO'
'in' is the link.
Can you help me to learn more?
(Other definitions for ufo that I've seen before include "Space enigma" , "Potential flying saucer" , "Mysterious craft" , "Something mysterious" , "ET's vehicle?" .)