Second outsize European deer (5)
I believe the answer is:
moose
'deer' is the definition.
(moose is a kind of deer)
'second outsize european' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'outsize' becomes 'os'.
'european' becomes 'E' (abbreviation e.g. EU).
'mo'+'os'+'e'='MOOSE'
(Other definitions for moose that I've seen before include "US deer with enormous antlers" , "Large northern deer" , "Large US deer" , "Ungulate" , "beasts" .)