Day before critical for baboon, say? (6)
I believe the answer is:
monkey
'baboon say?' is the definition.
(baboon is a kind of monkey)
'day before critical' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'before' says to put letters next to each other.
'critical' becomes 'key' (I've seen this in another clue).
'mon'+'key'='MONKEY'
'for' is the link.
(Other definitions for monkey that I've seen before include "Any of various long-tailed primates" , "Medium-sized primate" , "Chile pine (the Araucaria tree)" , "Primate; five hundred (slang)" , "Mischievous type" .)