Feet initially incised by American surgeon (5)
I believe the answer is:
iambs
'feet' is the definition.
(poetry rhythm)
'initially incised by american surgeon' is the wordplay.
'initially' says to take the initial letters.
'by' means one lot of letters go next to another.
'american' becomes 'am' (abbreviation in some contexts).
'surgeon' becomes 'bs'.
The first letter of 'incised' is 'i'.
'i'+'am'+'bs'='IAMBS'
(Other definitions for iambs that I've seen before include "bits of poetry" , "feet" .)