A force within the TA on my year off (8)
I believe the answer is:
yeomanry
'a force within the ta' is the definition.
(Royal Yeomanry in British Territorial Army)
'on my year off' is the wordplay.
'off' is an anagram indicator.
'on'+'my'+'year'='onmyyear'
'onmyyear' with letters rearranged gives 'YEOMANRY'.
(Other definitions for yeomanry that I've seen before include "Old cavalry volunteer force" , "Guards" , "Group of small estate owners (historical)" , "Originally, cavalry force of smallholders" , "A body of smaller freeholders (hist.)" .)