Highest point — unplayable tennis serve Mike welcomed (4)
I believe the answer is:
acme
'highest point' is the definition.
(I've seen this before)
'unplayable tennis serve mike welcomed' is the wordplay.
'unplayable tennis serve' becomes 'ace' (tennis serve which an opponent can't return).
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'welcomed' indicates putting letters inside (inserted letters are welcomed in).
'ace' enclosing 'm' is 'ACME'.
(Other definitions for acme that I've seen before include "crown" , "highest position" , "Summit, crowning point" , "Came awkwardly to the highest point" , "Culminating point" .)