Opening's briefly unpleasant in rain (6)
I believe the answer is:
wicket
'opening's' is the definition.
(wicket is a kind of opening)
'briefly unpleasant in rain' is the wordplay.
'briefly' means to remove the last letter.
'unpleasant' becomes 'icky' (icky can mean disgusting).
'in' indicates putting letters inside.
'rain' becomes 'wet' (I've seen this in other clues).
'icky' with its final letter removed is 'ick'.
'ick' put into 'wet' is 'WICKET'.
(Other definitions for wicket that I've seen before include "that first-class delivery may take" , "Stump" , "Three stumps and two bails" , "Stumps and bails construction - success for a bowler" , "playing area" .)