Turn fifty, pursuing a purpose (4)
I believe the answer is:
goal
'purpose' is the definition.
(I've seen this in another clue)
'turn fifty pursuing a' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'fifty' becomes 'l' (Roman numeral).
'pursuing' means one lot of letters go next to another.
'l' after 'a' is 'al'.
'go'+'al'='GOAL'
(Other definitions for goal that I've seen before include "Aim - striker's target" , "Soccer achievement" , "A score in soccer" , "Target, objective" , "Aim, desired result" .)