Fool has trouble following attack (6)
I believe the answer is:
assail
'attack' is the definition.
(I know that attack can be written as assail)
'fool has trouble' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'has' says to put letters next to each other.
'trouble' becomes 'ail' (synonyms).
'ass'+'ail'='ASSAIL'
'following' acts as a link.
(Other definitions for assail that I've seen before include "Make violent attack on" , "Criticize" , "Maybe mug" , "Attack violently and persistently" , "Trouble" .)