Modify fittingly (5)
I believe the answer is:
adapt
'modify fittingly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adapt that I've seen before include "Convert to another use or situation" , "Modify; acclimatise" , "Change to suit a different purpose" , "Make fit" , "Change to suit new circumstances" .)