One held the string, in the old days (4)
I believe the answer is:
once
'in the old days' is the definition.
(I've seen this before)
'one held the string' is the wordplay.
'held' is an insertion indicator.
'the string' becomes 'c' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' enclosing 'c' is 'ONCE'.
Can you help me to learn more?
(Other definitions for once that I've seen before include "One-time - one time" , "Cone (anag.)" , "Quick examination" , "A single time" , "'... bitten, twice shy'" .)