Heavenly letter in Gaelic, possibly (7)
I believe the answer is:
angelic
'heavenly' is the definition.
(relating to angels or heaven)
'letter in gaelic possibly' is the wordplay.
'letter' becomes 'n'.
'in' indicates putting letters inside.
'possibly' is an anagram indicator.
'gaelic' anagrammed gives 'agelic'.
'n' put inside 'agelic' is 'ANGELIC'.
(Other definitions for angelic that I've seen before include "Virtuous" , "Very well behaved" , "The host's" , "Heavenly - entrancing" , "Exceptionally innocent" .)