Astronomical instrument still initially surviving (7)
'astronomical instrument' is the definition.
'sextant' can be an answer for 'instrument' (sextant is a kind of instrument). I am not certain of the 'astronomical' bit.
'still initially surviving' is the wordplay.
'initially' suggests taking the first letters.
'surviving' becomes 'extant' ('extant' can be similar in meaning to 'surviving').
The initial letter of 'still' is 's'.
Can you help me to learn more?
(Other definitions for sextant that I've seen before include "Mariner's instrument", "Navigational aid", "Nautical instrument for measuring angular distances", "Measuring instrument on board", "Sighting instrument".)