Some worried about old animal (5)
I believe the answer is:
moose
'animal' is the definition.
(type of animal)
'some worried about old' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'some' is an anagram of 'mose'.
'mose' enclosing 'o' is 'MOOSE'.
(Other definitions for moose that I've seen before include "Large N American deer like European elk" , "Ungulate" , "beasts" , "Animal" , "North American deer" .)