Weave poorly under back edges of garret window (5)
I believe the answer is:
twill
'weave' is the definition.
(I know that twill is a type of weave)
'poorly under back edges of garret window' is the wordplay.
'poorly' becomes 'ill' (synonyms).
'under' means one lot of letters go next to another (below, in a down clue).
'back' says the letters should be written backwards.
'edges of garret window' becomes 'wt' (I can't explain this - if you can you should believe this answer much more).
'wt' reversed gives 'tw'.
'ill' put after 'tw' is 'TWILL'.
Can you help me to learn more?
(Other definitions for twill that I've seen before include "Cloth with diagonal ribs" , "Stuff" , "Fabric woven to have diagonal ribs" , "Diagonal-weave fabric" , "Strong ridged fabric" .)