Sailor runs into sea with little hesitation (7)
I believe the answer is:
mariner
'sailor' is the definition.
(mariner means a sailor)
'runs into sea with little hesitation' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'sea' becomes 'main' (synonyms).
'with' says to put letters next to each other.
'little hesitation' becomes 'er' (expression used when hesitating).
'r' going inside 'main' is 'marin'.
'marin'+'er'='MARINER'
(Other definitions for mariner that I've seen before include "Probe" , "Person sailing" , "'''The Rime of the Ancient ...'' (Coleridge) (7)'" , "Jack" , "Man of the sea" .)