Has to contain brief rash causing bother (6)
I believe the answer is:
harass
'bother' is the definition.
(I know that harass is a more specific form of bother)
'has to contain brief rash' is the wordplay.
'to contain' means one lot of letters goes inside another.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'rash' with its final letter taken away is 'ras'.
'has' placed around 'ras' is 'HARASS'.
'causing' acts as a link.
(Other definitions for harass that I've seen before include "Continually torment" , "Trouble constantly" , "press" , "Pester constantly" , "bug" .)