Old blunder covered up by TA soldiers (5,5)
I believe the answer is:
other ranks
'soldiers' is the definition.
(I've seen this before)
'old blunder covered up by ta' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'blunder' becomes 'err' (I've seen this in another clue).
'covered up by' is an insertion indicator.
'ta' becomes 'thanks' (I've seen this before).
'err' placed inside 'thanks' is 'therranks'.
'o'+'therranks'='OTHER RANKS'
(Other definitions for other ranks that I've seen before include "Most of the men who fight" , "Enlisted troops" , "soldiers" , "Non-commissioned armed forces members" , "more rows" .)