Mournful English adopted by version of Gaelic (7)
I believe the answer is:
elegiac
'mournful' is the definition.
(synonym)
'english adopted by version of gaelic' is the wordplay.
'english' becomes 'e' (abbreviation).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'version of' is an anagram indicator.
'gaelic' is an anagram of 'elgiac'.
'e' inserted into 'elgiac' is 'ELEGIAC'.
(Other definitions for elegiac that I've seen before include "Wistful, mournful" , "of valedictory works" , "Poetic, mournful" , "Mournful as in Gray's poem" , "Funereal" .)