Players notice team's leader training inside (6)
I believe the answer is:
septet
'players' is the definition.
(musical group)
'notice team's leader training inside' is the wordplay.
'notice' becomes 'see' (I've seen this before).
'leader' says to take the initial letters.
'training' becomes 'pt' (**).
'inside' means one lot of letters goes inside another.
The first letter of 'teams' is 't'.
'see'+'t'='seet'
'seet' placed around 'pt' is 'SEPTET'.
(Other definitions for septet that I've seen before include "Seven-piece band" , "Group of seven players" , "Musical composition written for seven performers" , "Band of seven players" , "Heptad" .)