Girl initially wearing messy anorak and old jumper (8)
I believe the answer is:
kangaroo
'jumper' is the definition.
(thesaurus)
'girl initially wearing messy anorak and old' is the wordplay.
'initially' indicates taking the first letters.
'wearing' is an insertion indicator.
'messy' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The initial letter of 'girl' is 'g'.
'anorak' anagrammed gives 'kanaro'.
'g' put into 'kanaro' is 'kangaro'.
'kangaro'+'o'='KANGAROO'
(Other definitions for kangaroo that I've seen before include "Jumping animal" , "Jumper of grey or red" , "Jumping marsupial" , "Marsupial from down-under" , "Bounding marsupial" .)