An outdoor event Pat and Gerry organised (6,5)
I believe the answer is:
garden party
'an outdoor event' is the definition.
(event in a garden)
'pat and gerry organised' is the wordplay.
'organised' indicates anagramming the letters.
'pat'+'and'+'gerry'='patandgerry'
'patandgerry' anagrammed gives 'GARDEN PARTY'.
(Other definitions for garden party that I've seen before include "Lawn event" , "Formal festive outdoor occasion" , "Social occasion at Buckingham Palace, perhaps" , "Alfresco social event" , "Buckingham Palace event" .)