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' suggests halving the letters.
'i' becomes 'compiler' (the person who set this crossword).
'see' becomes 'C' (the word for the letter, according to Chambers).
'compiler' halved is 'iler'.
'iler' backwards is 'reli'.
'reli'+'c'='RELIC'
'may serve as' acts as a link.
This may not be right. 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" .)