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' is a reversal indicator.
'50' suggests halving the letters.
'i' becomes 'compiler' (the person that set this crossword).
'see' becomes 'C' (the word for the letter, according to Chambers).
'compiler' halved is 'iler'.
'iler' written backwards gives 'reli'.
'reli'+'c'='RELIC'
'may serve as' is the link.
This may not be correct. Some or all of it may be part of 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" .)