'Playground rugby' -- I finally put sign up (7)
I believe the answer is:
recruit
'sign up' is the definition.
(I've seen this in another clue)
'playground rugby i finally put' is the wordplay.
'playground' becomes 'rec' (short for recreation ground).
'rugby' becomes 'RU'.
'finally' suggests the final letters.
The final letter of 'put' is 't'.
'rec'+'ru'+'i'+'t'='RECRUIT'
(Other definitions for recruit that I've seen before include "Enlist (someone) into the armed forces" , "Newly enlisted soldier etc" , "New member of army etc" , "Trainee" , "Sign up" .)