Limitless tax crushes son (4)
I believe the answer is:
vast
'limitless' is the definition.
(synonyms)
'tax crushes son' is the wordplay.
'tax' becomes 'VAT' (UK Value Added Tax).
'crushes' means one lot of letters goes inside another.
'son' becomes 's' (genealogical abbreviation for son).
'vat' going around 's' is 'VAST'.
(Other definitions for vast that I've seen before include "Gigantic" , "Boundless" , "Extremely large or extensive" , "Astronomical" , "Unusually great" .)