Sailors about to visit Cracow on vacation (4)
I believe the answer is:
crew
'sailors' is the definition.
(as in a ship's crew)
'about to visit cracow on vacation' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to visit' is an insertion indicator (inserted letters visit others).
'on vacation' says to hollow out the word (remove centre letters) (after the inside letters have vacated the word).
'cracow' with its middle taken out is 'cw'.
're' going within 'cw' is 'CREW'.
(Other definitions for crew that I've seen before include "Staff of boat" , "Those who man a ship or aircraft" , "On board first (4)" , "Company working together" , "Ship's operators" .)