Main suspect eludes arrest initially (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'suspect eludes arrest initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'suspect eludes arrest' is 'SEA'.
(Other definitions for sea that I've seen before include "E.g Aral or Weddell" , "All at . . . - totally confused" , "Large body of salt water" , "Cause of flooding" , "Marine" .)