Party ultimately hosted by Robert, with group of lads (3,4)
I believe the answer is:
boy band
'group of lads' is the definition.
'boy band' can be an answer for 'group' (I have seen 'Young group ' mean 'boy band' so perhaps 'group' could also mean 'boy band'). I am unsure of the 'of lads' bit.
'party ultimately hosted by robert with' is the wordplay.
'ultimately' indicates one should take the final letters.
'hosted by' is an insertion indicator.
'robert' becomes 'bob' (short for Robert).
'with' becomes 'and'.
The last letter of 'party' is 'y'.
'y' put inside 'bob' is 'boyb'.
'boyb'+'and'='BOY BAND'
Can you help me to learn more?
(Other definitions for boy band that I've seen before include "performers" , "One Direction, for example?" , "Group of young males" .)