An annoying summer visitor's back in minimalist garment (5)
I believe the answer is:
tanga
'minimalist garment' is the definition.
(type of undergarment)
'an annoying summer visitor's back' is the wordplay.
'an' becomes 'a'.
'annoying summer visitor' becomes 'gnat' (pest in summer).
'back' says the letters should be written in reverse.
'a'+'gnat'='agnat'
'agnat' back-to-front is 'TANGA'.
'in' acts as a link.
(Other definitions for tanga that I've seen before include "Briefs of two small panels connected by strings" , "Briefs not covering hips" , "skimpy garment" , "Very brief bikini" , "underwear" .)