Airline's mechanics initially work well (5)
I believe the answer is:
amply
'well' is the definition.
(thesaurus)
'airline's mechanics initially work' is the wordplay.
'initially' says to take the initial letters.
'work' becomes 'ply' (I've seen this in other clues).
The initial letters of 'airlines mechanics' is 'am'.
'am'+'ply'='AMPLY'
(Other definitions for amply that I've seen before include "Sufficiently" , "Fully successful" , "In a plentiful manner" , "more than adequately" , "in accordance with liberal policy" .)