Fit amateur lacking experience falls short (5)
I believe the answer is:
agree
'fit' is the definition.
(synonyms)
'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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'am' with its last 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?" .)