Fully able to slot in nicely (3)
I believe the answer is:
fit
'fully able' is the definition.
'fit' can be an answer for 'able' (similar in meaning). I am not sure about the 'fully' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fit that I've seen before include "Suit" , "Be the right size or shape" , "In trim" , "Physically well" , "Sound in body or suitable" .)