Fine hose following back of thigh (8)
I believe the answer is:
hairline
'fine' is the definition.
(hairline can mean very fine or thin)
'hose following back of thigh' is the wordplay.
'hose' becomes 'airline' (pipe supplying air).
'following' means one lot of letters go next to another.
'back of' says to take the final letters.
The final letter of 'thigh' is 'h'.
'airline' after 'h' is 'HAIRLINE'.
(Other definitions for hairline that I've seen before include "Very narrow opening" , "Of a crack, very thin" , "A very narrow crack, a little hirsute?" , "extremely thin" , "Type of crack" .)