Worthless supporter climbing on top of fence (4)
I believe the answer is:
naff
'worthless' is the definition.
(naff can mean of low quality)
'supporter climbing on top of fence' is the wordplay.
'supporter' becomes 'fan' (fan is a kind of supporter).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'on top of' suggests taking the first letters.
The first letter of 'fence' is 'f'.
'fan' reversed gives 'naf'.
'naf'+'f'='NAFF'
(Other definitions for naff that I've seen before include "socially awkward" , "In bad taste" , "Inferior - vulgar" , "pants" , "Lacking taste or style" .)