Drab debutante dancing first part of rumba routine (5,3,6)
I believe the answer is:
bread and butter
'routine' is the definition.
(bread and butter can mean routine or typical)
'drab debutante dancing first part of rumba' is the wordplay.
'dancing' indicates an anagram.
'first part of' suggests taking the first letters.
The first letter of 'rumba' is 'r'.
'drab'+'debutante'='drabdebutante'
'drabdebutante' is an anagram of 'breadandbutte'.
'breadandbutte'+'r'='BREAD-AND-BUTTER'
(Other definitions for bread and butter that I've seen before include "Main source of income" , "Means of support" , "A crust" , "earned?" , "basic course" .)