Frequently dull without leader (5)
I believe the answer is:
often
'frequently' is the definition.
('often' can be a synonym of 'frequently')
'dull without leader' is the wordplay.
'dull' becomes 'soften'.
'without leader' means to remove the first letter.
'soften' with its first letter removed is 'OFTEN'.
(Other definitions for often that I've seen before include "Multiply" , "A lot" , "Turn away" , "Frequently, at short intervals" , "Frequently, habitually" .)