The most common and most likely answer to this crossword clue is the 4 letter word FYRD
F
Y
R
D
Buddy explains!
The straight part
We know that "old fighting force" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old fighting force = fyrd.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.
What does FYRD mean?
FYRD noun- In early Anglo-Saxon times, an army that was mobilized from freemen to defend their shire, or from select representatives to join a royal expedition.