I had to go back to get a letter (4)

I believe the answer is:

iota

'letter' is the definition.
(iota is a kind of letter)

'i had to go back to get a' is the wordplay.
'had' says to put letters next to each other.
'to go back' says the letters should be written in reverse.
'get' is a charade indicator (letters next to each other).
'to' in reverse letter order is 'ot'.
'i'+'ot'+'a'='IOTA'

(Other definitions for iota that I've seen before include "Trace" , "Smidgeon" , "Tiny or scarcely detectable amount" , "Tiny bit" , "Greek letter - small quantity" .)

I've seen this clue in the Evening Standard.

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