The most common and most likely answer to this crossword clue is the 6 letter word ATTLEE
I've seen this crossword clue in Independent
A
T
T
L
E
E
More clues leading to the result ATTLEE
- Head lost in fight with English PM
- PM ready to drive off on course, keeping left?
- Hostilities having start delayed by English politician
- Former Prime Minister perhaps steers European leader to expel Conservative
- Former PM and Attorney 19
- PM losing head in conflict with European
- Ex-PM, captured during battle, escaped
- 12 Former PM featured in Seattle exhibition
- PM in general beyond a gutless twit
- Fight to oust second-class English PM
Buddy explains!

The straight part
We know that "Former Prime Minister" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case former prime minister = attlee.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.