Young female horse sick in outskirts of Fowey (5)
I believe the answer is:
filly
'young female horse' is the definition.
(I've seen this before)
'sick in outskirts of fowey' is the wordplay.
'sick' becomes 'ill' (synonyms).
'in' means one lot of letters goes inside another.
'outskirts of' means to remove the middle letters (only letters on the outskirts).
'fowey' with its middle removed is 'fy'.
'ill' placed within 'fy' is 'FILLY'.
(Other definitions for filly that I've seen before include "Young female horse" , "Female horse under four years old" , "Young mare" .)