Knowing where to spend a penny, then another (2,3,4)
I believe the answer is:
in the loop
'knowing' is the definition.
(in the loop means informed about something)
'to spend a penny then another' is the wordplay.
I cannot quite see how this works, but
'penny' could be 'p' (short for pence) and 'p' is present in the answer.
an anagram of 'then' is 'nthe' which is present in the answer.
'another' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
This explanation may well be incorrect...
'where' is the link.
(Other definitions for in the loop that I've seen before include "Well informed" , "briefed?" , "kept informed" .)