Study in small hotel, say, sustained income (5,3,6)
I believe the answer is:
bread and butter
'sustained income' is the definition.
'study in small hotel say' is the wordplay.
'study' becomes 'read' (synonyms).
'in' is an insertion indicator.
'small hotel' becomes 'bandb' (abbreviation for bed and breakfast hotel).
'say' becomes 'utter' (I've seen this before).
'read' placed into 'bandb' is 'breadandb'.
'breadandb'+'utter'='BREAD-AND-BUTTER'
(Other definitions for bread and butter that I've seen before include "basic course" , "sandwiches?" , "Subsistence" , "pudding" , "rather monotonous diet" .)