Invader has trouble crossing dry area (6)
I believe the answer is:
attila
'invader' is the definition.
(Attila the Hun)
'trouble crossing dry area' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'crossing' means one lot of letters goes inside another.
'dry' becomes 'tt' (abbreviation for teetotal).
'area' becomes 'a' (maths abbreviation).
'ail' placed around 'tt' is 'attil'.
'attil'+'a'='ATTILA'
'has' is the link.
(Other definitions for attila that I've seen before include "Brutal Hunnish king" , "Devastating ruler" , "Fifth-century king of the Huns" , "Well-known Hun" , "A very great Hun of old" .)