As tax may be, having set up island — old money welcome (8)
I believe the answer is:
evadable
'as tax may be' is the definition.
(as in tax evasion)
'set up island old money welcome' is the wordplay.
'set up' says the letters should be written backwards (in a down clue, letters go up).
'island' becomes 'elba' (Italian island).
'old money' becomes 'd'.
'welcome' becomes 'ave' ('ave' is a Latin greeting).
'elba'+'d'+'ave'='elbadave'
'elbadave' in reverse letter order is 'EVADABLE'.
'having' is the link.
(Other definitions for evadable that I've seen before include "Such as can be avoided" , "Possible to avoid" .)