… creature initially presenting very big problem (6)
I believe the answer is:
possum
'creature' is the definition.
(possum is a kind of creature)
'initially presenting very big problem' is the wordplay.
'initially' suggests taking the first letters.
'very big' becomes 'os' (abbreviation for outsize).
'problem' becomes 'sum' (arithmetic problem).
The initial letter of 'presenting' is 'p'.
'p'+'os'+'sum'='POSSUM'
(Other definitions for possum that I've seen before include "Marsupial - play it to feign sleep" , "The Australasian phalanger" , "American marsupial (that plays dead?)" , "Small furry Australian marsupial" , "Tree-dwelling Australasian marsupial" .)