Boring man promises to pay (7)
I believe the answer is:
tedious
'boring' is the definition.
(I know that boring can be written as tedious)
'man promises to pay' is the wordplay.
'man' becomes 'ted' (Ted is a man's name).
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'ted'+'ious'='TEDIOUS'
(Other definitions for tedious that I've seen before include "not exciting" , "Outside (anag.)" , "Banal" , "Humdrum" , "Tiresome, boring" .)