Bishop isn't dignified gatecrashing function (5,9)
I believe the answer is:
saint augustine
'bishop' is the definition.
(3rd- and 4th-century theologian and bishop)
'isn't dignified gatecrashing function' is the wordplay.
'isn't' becomes 'aint' (non-standard version of 'isn't').
'dignified' becomes 'august' (I've seen this before).
'gatecrashing' is an insertion indicator.
'function' becomes 'sine' (mathematical function).
'aint'+'august'='aintaugust'
'aintaugust' going inside 'sine' is 'SAINT AUGUSTINE'.
(Other definitions for saint augustine that I've seen before include "First archbishop of Canterbury" , "Old Christian" .)