Mop sure to be used by big chief (7)
I believe the answer is:
supremo
'big chief' is the definition.
'supremo' can be an answer for 'chief' (I've seen this before). I'm unsure of the 'big' bit.
'mop sure to be used' is the wordplay.
'to be used' indicates an anagram.
'mop'+'sure'='mopsure'
'mopsure' is an anagram of 'SUPREMO'.
'by' is the link.
Can you help me to learn more?
(Other definitions for supremo that I've seen before include "Person in overall charge, really the top man" , "Person in overall charge, big boss" , "Person invested with great authority" , "C-in-C" , "big Daddy" .)