Tick seen on old south-east American deer (5)
I believe the answer is:
moose
'american deer' is the definition.
(I've seen this before)
'tick seen on old south-east' is the wordplay.
'tick' becomes 'mo' (both can mean a very short amount of time).
'seen on' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'south east' becomes 'se'.
'mo'+'o'+'se'='MOOSE'
(Other definitions for moose that I've seen before include "Large N American deer like European elk" , "Large US deer" , "North American elk" , "North American deer" , "Large northern deer" .)