The old, old fellow’s line for old British cavalry force (8)
I believe the answer is:
yeomanry
'old british cavalry force' is the definition.
'yeomanry' can be an answer for 'force' (yeomanry is a kind of force). I am not sure about the 'old british cavalry' bit.
'the old old fellow's line' is the wordplay.
'the old' becomes 'ye'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'man' (I've seen this before).
'line' becomes 'ry' (abbreviation for railway).
'ye'+'o'+'man'+'ry'='YEOMANRY'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for yeomanry that I've seen before include "Group of bodyguards" , "Class of freeholders (historical)" , "A body of smaller freeholders (hist.)" , "Group of small estate owners (historical)" , "chaps on estates, as before" .)