Surgery has problem restraining very big animal (7)
I believe the answer is:
opossum
'animal' is the definition.
(opossum is a kind of animal)
'surgery has problem restraining very big' is the wordplay.
'surgery' becomes 'op' (short for operation).
'has' says to put letters next to each other.
'problem' becomes 'sum' (arithmetic problem).
'restraining' is an insertion indicator.
'very big' becomes 'os' (abbreviation for outsize).
'op'+'sum'='opsum'
'opsum' going around 'os' is 'OPOSSUM'.
(Other definitions for opossum that I've seen before include "Small Australian marsupial" , "Large marsupial" , "Thick-furred marsupial of the Americas" , "American and Australian marsupial, often abbreviated" , "New World marsupial" .)