Language if rash almost incomprehensible (5)
I believe the answer is:
farsi
'language' is the definition.
(I know that Farsi is a natural language)
'if rash almost incomprehensible' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'incomprehensible' indicates anagramming the letters (an incomprehensible jumbling of the letters).
'if'+'rash'='ifrash'
'ifrash' with its final letter removed is 'ifras'.
'ifras' anagrammed gives 'FARSI'.
(Other definitions for farsi that I've seen before include "Iranian language, also widely spoken in Afghanistan" , "Modern Persian lang." , "Language of Iran" , "foreign language" , "Iran's official language" .)