Adult crowds half ignored barmy circus performers (8)
I believe the answer is:
acrobats
'circus performers' is the definition.
(I've seen this before)
'adult crowds half ignored barmy' is the wordplay.
'adult' becomes 'a' (abbreviation).
'half ignored' indicates taking half.
'barmy' becomes 'bats' (synonyms).
'crowds' cut in half is 'cro'.
'a'+'cro'+'bats'='ACROBATS'
(Other definitions for acrobats that I've seen before include "Athletic circus performers" , "Circus peformers" , "Circus athletes or gymnasts" , "Gymnasts; tumblers" , "versatile performers" .)