Maybe Irish agile playing at speed of light (6)
I believe the answer is:
gaelic
'maybe irish' is the definition.
(Irish Gaelic)
'agile playing at speed of light' is the wordplay.
'playing' indicates an anagram.
'at' says to put letters next to each other.
'speed of light' becomes 'c' (as in E=mc2).
'agile' anagrammed gives 'gaeli'.
'gaeli'+'c'='GAELIC'
(Other definitions for gaelic that I've seen before include "Characteristic of the Celts" , "Irish language or Celtic language of Scotland" , "Language of the Celts" , "Speech" , "tongue" .)