In a rut, a bad score means heavy defeat! (4)
I believe the answer is:
rout
'heavy defeat' is the definition.
(I've seen this before)
'in a rut a bad score' is the wordplay.
'in' means one lot of letters goes inside another.
'a bad score' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rut' enclosing 'o' is 'ROUT'.
'means' is the link.
Can you help me to learn more?
(Other definitions for rout that I've seen before include "Put to flight, scatter" , "Whitewash" , "clobber" , "Total defeat in battle" , "Overwhelming defeat or disorderly retreat" .)