Mathematician somewhere in Italy heading for Genoa (6)
I believe the answer is:
turing
'mathematician' is the definition.
(Alan Turing)
'somewhere in italy heading for genoa' is the wordplay.
'somewhere in italy' becomes 'Turin' (Italian city).
'heading for' indicates taking the first letters.
The initial letter of 'genoa' is 'g'.
'turin'+'g'='TURING'
(Other definitions for turing that I've seen before include "British mathematician" , "With his machine, he broke German codes during World War II" , "Alan --, twentieth century English mathematician" , "Alan -, British computer pioneer" .)