Total area occupied by uncommon rat (according to Holmes, at least) (7)
I believe the answer is:
sumatra
'holmes at least' is the definition.
I can't tell whether this defines the answer.
'total area occupied by uncommon rat' is the wordplay.
'total' becomes 'sum' (synonyms).
'area' becomes 'a' (maths abbreviation).
'occupied by' indicates putting letters inside (inserted letters occupy or enter the word).
'uncommon' indicates anagramming the letters.
'sum'+'a'='suma'
'rat' with letters rearranged gives 'atr'.
'suma' placed around 'atr' is 'SUMATRA'.
'according to' acts as a link.
Can you help me to learn more?
(Other definitions for sumatra that I've seen before include "Some land" , "Large Indonesian island" , "Second largest Indonesian island" , "part of Indonesia" , "A US mart on an Indonesian island?" .)