Was their job to weaken enemy strength? (7)
I believe the answer is:
sappers
'was their job' is the definition.
I can't judge whether this definition defines the answer.
'weaken enemy strength?' is the wordplay.
'weaken' becomes 'sap' (I've seen this in another clue).
'enemy strength?' becomes 'pers' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sap'+'pers'='SAPPERS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for sappers that I've seen before include "Military engineers" , "Royal Engineers" .)