Surgeon's first working rule: man, I like the aortic valve! (9)

I believe the answer is:

semilunar

'like the aortic valve' is the definition.
(the aortic valve is crescent shaped)

'surgeon's first working rule man i' is the wordplay.
'surgeon's first' becomes 's' (1st letter of 'surgeon').
'working' is an anagram indicator.
'rule'+'man'+'i'='rulemani'
'rulemani' anagrammed gives 'emilunar'.
's'+'emilunar'='SEMILUNAR'

(Another definition for semilunar that I've seen is " like half a grapefruit?".)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app