Bill used regularly to be sad (4)
I believe the answer is:
blue
'be sad' is the definition.
('blue' can be similar in meaning to 'sad')
'bill used regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'bill'+'used'='billused'
The alternate letters of 'billused' are 'BLUE'.
'to' acts as a link.
(Other definitions for blue that I've seen before include "The colour of melancholy" , "Primary colour" , "Sad; butterfly" , "Sad, melancholy" , "Australian redhead" .)