Rhyming slang, say, is included by US author, male (9)

'rhyming slang say' is the definition.
(word or way of speaking associated with London)

'is included by us author male' is the wordplay.
'included by' indicates putting letters inside.
'us author' becomes 'london' (novelist Jack London).
'male' becomes 'm' (common abbreviation).
'london'+'m'='londonm'
'is' placed within 'londonm' is 'LONDONISM'.