Setter's perhaps half-straight feet? (5)
I believe the answer is:
iambi
'feet?' is the definition.
(plural of iamb, a type of metrical foot in poetry)
'setter's perhaps half-straight' is the wordplay.
'setter's' becomes 'I am' (to the crossword setter, 'setter is' means 'I am').
'perhaps half straight' becomes 'bi' (bisexual).
'iam'+'bi'='IAMBI'
(Other definitions for iambi that I've seen before include "Short-long metrical feet" , "Metrical patterns" , "Metrical units with alternating long and short syllables" , "Poetic feet" , "Feet of two syllables (one short and one long)" .)