Taking the most pessimistic view, a team's leader before defeat (2,5)
I believe the answer is:
at worst
'taking the most pessimistic view' is the definition.
(I've seen this before)
'a team's leader before defeat' is the wordplay.
'leader' suggests taking the first letters.
'before' means one lot of letters go next to another.
'defeat' becomes 'worst' (worsting is a kind of defeating).
The initial letter of 'teams' is 't'.
'a'+'t'+'worst'='AT WORST'
(Other definitions for at worst that I've seen before include "in the most serious state" , "Taking the most pessimistic view" , "Taking blackest view" , "In the most negative circumstances" , "most pessimistically" .)