Big swelling on the end of a finger (6)
I believe the answer is:
bumper
'big' is the definition.
(I've seen this before)
'swelling on the end of a finger' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found within the answer.
The remaining letters 'bum' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bumper that I've seen before include "It's a car fender in America" , "Something big" , "kind of car" , "Big - fender" , "bouncer" .)