About 50, I see, may serve as a reminder (5)
I believe the answer is:
relic
'a reminder' is the definition.
(thesaurus)
'about 50 i see' is the wordplay.
'about' shows that the letters should be reversed in order.
'50' means to take half the letters.
'i' becomes 'compiler' (the person who set this crossword).
'see' becomes 'C' (the word for the letter, according to Chambers).
'compiler' cut in half is 'iler'.
'iler' written backwards gives 'reli'.
'reli'+'c'='RELIC'
'may serve as' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for relic that I've seen before include "E,g. dead saint's bone" , "item of historic interest" , "for remembrance" , "Trace" , "Ancient artefact" .)