Is sleuth returning to open certain letters? ... (8)
I believe the answer is:
ellipsis
'is sleuth returning to open certain letters?' is the definition.
I can't judge whether this definition defines the answer.
'is sleuth returning to open certain letters?' is the wordplay.
'sleuth' becomes 'pi' (private investigator).
'returning' says the letters should be written in reverse.
'to open' indicates putting letters inside (some letters open a gap in others).
'certain letters?' becomes 'ells' (ell is the name for the letter L).
'is'+'pi'='ispi'
'ispi' written backwards gives 'ipsi'.
'ipsi' put into 'ells' is 'ELLIPSIS'.
(Other definitions for ellipsis that I've seen before include "dotty marking" , "Omission of words from speech or writing" , "Set of three dots in a text indicating an omission" , "dotty row" , "Omission of parts of words, in poetry say" .)