Former Gaelic clothes woven with loom at first (9)
I believe the answer is:
erstwhile
'former' is the definition.
('erstwhile' can be a synonym of 'former')
'gaelic clothes woven with loom at first' is the wordplay.
'gaelic' becomes 'erse' ('Erse' can be a synonym of 'gaelic').
'clothes' is an insertion indicator.
'woven' indicates an anagram.
'at first' says to take the initial letters.
The initial letter of 'loom' is 'l'.
'with'+'l'='withl'
'withl' with letters rearranged gives 'twhil'.
'erse' placed around 'twhil' is 'ERSTWHILE'.
(Other definitions for erstwhile that I've seen before include "Former - list where (anag)" , "Former, previous" , "In the past" , "Formerly" , "Quondam" .)