Youngsters getting in the Boar's Head by the back door! (5)
I believe the answer is:
brood
'youngsters' is the definition.
(thesaurus)
'head by the back door' is the wordplay.
'head' indicates taking the first letters.
'the back' is a reversal indicator.
The first letter of 'by' is 'b'.
'door' backwards is 'rood'.
'b'+'rood'='BROOD'
'getting in the boar's' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for brood that I've seen before include "Collection of chickens" , "Think resentfully about family of children?" , "Family of young birds" , "Clutch" , "Think resentfully - about young birds?" .)