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' indicates putting letters inside.
'son' becomes 's' (genealogical abbreviation for son).
'vat' enclosing 's' is 'VAST'.
(Other definitions for vast that I've seen before include "Gigantic" , "Boundless" , "Extremely large or extensive" , "Astronomical" , "Unusually great" .)