Eyeshade briefly seen on professional rider (7)
I believe the answer is:
proviso
'rider' is the definition.
(I've seen this before)
'eyeshade briefly seen on professional' is the wordplay.
'eyeshade' becomes 'visor' (synonyms).
'briefly' means to remove the last letter.
'seen on' is a charade indicator (letters next to each other).
'professional' becomes 'pro' (abbreviation).
'visor' with its last letter removed is 'viso'.
'viso' after 'pro' is 'PROVISO'.
(Other definitions for proviso that I've seen before include "Requirement" , "Clause in contract" , "Rider" , "Condition, qualification" , "Condition of stipulation of an agreement" .)