Siamese perhaps approximately on time (3)
I believe the answer is:
cat
'siamese perhaps' is the definition.
(siamese is a kind of cat)
'approximately on time' is the wordplay.
'approximately' becomes 'ca' (abbreviation for circa).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'ca'+'t'='CAT'
(Other definitions for cat that I've seen before include "Tiger, for instance" , "Lash < jazz fan" , "Domestic animal" , "Eg, tiger, lion" , "Feline - whip" .)