Large Greek with large son (5)
I believe the answer is:
gross
'large' is the definition.
(similar in meaning)
'greek with large son' is the wordplay.
'greek' becomes 'gr' (abbreviation for Greek).
'with' says to put letters next to each other.
'large' becomes 'os' (abbreviation for oversize).
'son' becomes 's' (genealogical abbreviation for son).
'gr'+'os'+'s'='GROSS'
(Other definitions for gross that I've seen before include "Repulsively large" , "Without deduction; flagrant" , "Description of crude" , "large total before deductions" , "figure well over a ton" .)