Long, large gent wandering Hungary's borders (7)
I believe the answer is:
lengthy
'long' is the definition.
(synonyms)
'large gent wandering hungary's borders' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'wandering' indicates anagramming the letters.
'hungary's' becomes 'hy' (I can't explain this - if you can you should give a lot more credence to this answer).
'borders' indicates putting letters inside (inserted letters are bordered by others).
'gent' anagrammed gives '[gent]'.
'[gent]'+'hy'='engthy'
'l' put within 'engthy' is 'LENGTHY'.
Can you help me to learn more?
(Other definitions for lengthy that I've seen before include "Protracted" , "Of great duration" , "Rather long" , "Prolix - tedious" , "Taking up a lot of time" .)