Diversification of retailers is certainly no lark (4,5)
I believe the answer is:
late riser
'certainly no lark' is the definition.
(larks are famously early risers)
'diversification of retailers' is the wordplay.
'diversification of' indicates an anagram (a varied or diverse form of the letters).
'retailers' is an anagram of 'LATE RISER'.
'is' is the link.
(Other definitions for late riser that I've seen before include "she spends half the day in bed" , "Lie-abed" .)