Followed short fellow, I understand (6)
I believe the answer is:
gotcha
'i understand' is the definition.
'followed short fellow' is the wordplay.
'followed' becomes 'got' (both can mean understood).
'short fellow' becomes 'cha' (I can't explain this - if you can you should give a lot more credence to this answer).
'got'+'cha'='GOTCHA'
(Other definitions for gotcha that I've seen before include "I have found you" , "You're caught - I understand you" , "You've been caught" , "cry of apprehension" , "claim to have made conquest?" .)