Sober Australian soldiers drinking tea mostly (7)
I believe the answer is:
austere
'sober' is the definition.
(I've seen this in another clue)
'australian soldiers drinking tea mostly' is the wordplay.
'australian' becomes 'aus' (common abbreviation).
'soldiers' becomes 're' (Royal Engineers).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'mostly' means to remove the last letter (most but not all of the word).
'aus'+'re'='ausre'
'tea' with its final letter removed is 'te'.
'ausre' going around 'te' is 'AUSTERE'.
(Other definitions for austere that I've seen before include "Severe, lacking comforts" , "Abstemious and grave" , "Stern or very plain" , "Severe; lacking luxuries" , "Strict in appearance" .)