Cryptic indication of Sally or Jack's rank (6,4)
I believe the answer is:
second mate
'cryptic indication' is the definition.
The definition suggests a singular noun which matches the answer.
'sally or jack's rank' is the wordplay.
I cannot really see how this works, but
'rank' could be 'second' (second is a kind of rank) and 'second' is located in the answer.
The remaining letters 'mate' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
(Other definitions for second mate that I've seen before include "Craftsman, perhaps" , "Naval officer" , "one involved in bigamy?" .)