Animal soldier held up in short period (5)
I believe the answer is:
tiger
'animal' is the definition.
(tiger is a type of animal)
'soldier held up in short period' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'held up' is a reversal indicator.
'in' means one lot of letters goes inside another.
'short period' becomes 'ter' (I've seen this in other clues).
'gi' back-to-front is 'ig'.
'ig' inserted within 'ter' is 'TIGER'.
(Other definitions for tiger that I've seen before include "''... ... burning bright, in the forests of the night'' (Blake)" , "audacious person" , "Beast - type of 1 across" , "Beast as Shere Khan" , "Dynamo" .)