Huge son stuck in tub (4)
I believe the answer is:
vast
'huge' is the definition.
(I know that huge can be written as vast)
'son stuck in tub' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'stuck in' means one lot of letters goes inside another.
'tub' becomes 'vat' (synonyms).
's' inserted inside 'vat' is 'VAST'.
(Other definitions for vast that I've seen before include "'Huge, immense (4)'" , "Boundless" , "Of very great extent" , "Gigantic" , "Extremely large or extensive" .)