When keeping a team in line (4)
I believe the answer is:
axis
'line' is the definition.
(axis is a kind of line)
'when keeping a team' is the wordplay.
'when' becomes 'as'.
'keeping' is an insertion indicator.
'a team' becomes 'xi' (eleven, a football team).
'as' placed around 'xi' is 'AXIS'.
'in' acts as a link.
(Other definitions for axis that I've seen before include "Line about which a body turns or a construction is symmetrical" , "Bloc" , "Feasible" , "Body's line of rotation" , "1936 alliance between Germany and Italy" .)