Become terrible Republican party spearhead for reform (2,4-6)
I believe the answer is:
go pear-shaped
'become terrible republican' is the definition.
I can't judge whether this definition defines the answer.
'party spearhead for reform' is the wordplay.
'party' becomes 'GOP' (GOP is an example).
'for reform' indicates an anagram (I've seen this in other clues).
'spearhead' with letters rearranged gives 'earshaped'.
'gop'+'earshaped'='GO PEAR-SHAPED'
Can you help me to learn more?
(Other definitions for go pear-shaped that I've seen before include "Get out of kilter (colloq.)" , "Run out of control" , "crash?" , "Turn out badly (in a fruity way?)" .)