Second person to mention a sheep (3)
I believe the answer is:
ewe
'a sheep' is the definition.
(a ewe is a female sheep)
'second person to mention' is the wordplay.
'second person' becomes 'you' (second person pronoun).
'to mention' indicates a 'sounds like' (homophone) clue (I've seen 'mentioned' mean this).
'you' is a homophone of 'EWE'.
(Other definitions for ewe that I've seen before include "Female in a fleece?" , "Ram's partner?" , "21a [SHEEP]" , "Lamb's dam" , "Dam of a sheep" .)