Removed 50 per cent of sign language (5)
I believe the answer is:
farsi
'language' is the definition.
(I know that Farsi is a natural language)
'removed 50 per cent of sign' is the wordplay.
'removed' becomes 'far' (similar in meaning).
'50 per cent of' means to take half the letters.
'sign' with half the letters taken is 'si'.
'far'+'si'='FARSI'
(Other definitions for farsi that I've seen before include "Iran's official language" , "Iranian language, also widely spoken in Afghanistan" , "foreign language" , "Modern Persian lang." , "Persian language" .)