Pioneers flying in a country raise toast (6,3,5)
I believe the answer is:
alcock and brown
'pioneers flying' is the definition.
(aviators John Alcock and Arthur Brown)
'in a country raise toast' is the wordplay.
'in' indicates putting letters inside.
'country' becomes 'land' ('land' can be a synonym of 'country').
'raise' becomes 'cock' (eg a dog can cock his leg).
'toast' becomes 'brown' (I've seen this before**).
'a'+'land'='aland'
'aland' enclosing 'cock' is 'alcockand'.
'alcockand'+'brown'='ALCOCK AND BROWN'
(Another definition for alcock and brown that I've seen is " They flew".)