As a part-time soldier, be wrong in rank (7)
I believe the answer is:
terrier
'as a part-time soldier' is the definition.
I can't judge whether this defines the answer.
'wrong in rank' is the wordplay.
'wrong' becomes 'err' (to err is make an error).
'in' is an insertion indicator.
'rank' becomes 'tier' (tier is a kind of rank).
'err' inserted inside 'tier' is 'TERRIER'.
'be' is the link.
Can you help me to learn more?
(Other definitions for terrier that I've seen before include "Small hunting dog" , "Persistent person" , "One who volunteers to fight" , "Small lively dog, may be Scotch" , "Bedlington - -" .)