Two birds at the outset naturally avoided tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(historical tax paid to the church)
'two birds at the outset' is the wordplay.
'two birds' means two substitutions for 'bird'.
'bird' becomes 'he' (I am not sure about this - if you are sure you should believe this answer much more).
'bird' becomes 'tit'.
'at the outset' means one lot of letters go next to another.
'he' put after 'tit' is 'TITHE'.
'naturally avoided' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tithe that I've seen before include "Tenth part - old tax" , "Levy, originally worth one tenth" , "Tax paid to church" , "One-tenth; church tax" , "Church payment" .)