Back in saddle, a pair of bachelors go out (3)
I believe the answer is:
ebb
'go out' is the definition.
(as in the tide going out)
'back in saddle a pair of bachelors' is the wordplay.
'a pair of bachelors' means two replacements for 'bachelor'.
'back in' suggests the final letters (the 'back' of the word).
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
The last letter of 'saddle' is 'e'.
'e'+'b'+'b'='EBB'
(Other definitions for ebb that I've seen before include "Fall back" , "Tidal retreat" , "Tidal outflow" , "Type of tide" , "Withdraw heartless" .)