Has to remove son - European is silent (4)
I believe the answer is:
mute
'is silent' is the definition.
(synonyms)
'has to remove son european' is the wordplay.
'has to' becomes 'must' (I've seen this before).
'remove' is a deletion indicator.
'son' becomes 's' (genealogical abbreviation for son).
'european' becomes 'E' (abbreviation e.g. EU).
'must' with 's' taken out is 'mut'.
'mut'+'e'='MUTE'
(Other definitions for mute that I've seen before include "having no voice" , "Device that softens the tone of a musical instrument" , "Speechless, soundless" , "Silent, not talking" , "Silencer" .)