Calm down if worried by a touch of biliousness (5)
I believe the answer is:
'calm down' is the definition.
(I've seen this in another clue)
'worried by a touch of biliousness' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'by' means one lot of letters go next to another.
'touch of' indicates taking the first letters.
The first letter of 'biliousness' is 'b'.
'ate' put after 'ab' is 'ABATE'.
'if' is the link.
(Other definitions for abate that I've seen before include "Relax" , "Diminish, mitigate" , "Moderate" , "Lessen - cease" , "Reduce" .)