Fit amateur lacking experience falls short (5)
I believe the answer is:
agree
'fit' is the definition.
(agreeing is a kind of fitting)
'amateur lacking experience falls short' is the wordplay.
'amateur' becomes 'am' (as in am-dram).
'lacking' means to remove the last letter (the word is incomplete or lacking).
'experience falls short' becomes 'gree' (I can't justify this - if you can you should give a lot more credence to this answer).
'am' with its final letter taken away is 'a'.
'a'+'gree'='AGREE'
Can you help me to learn more?
(Other definitions for agree that I've seen before include "Concur - correspond" , "Go along with" , "See eye to eye" , "Think alike" , "chime?" .)