Still surviving on board ship, they may be needed (8)
I believe the answer is:
sextants
'they may be needed' is the definition.
The definition suggests an adverb but the answer is not.
'still surviving on board ship' is the wordplay.
'still surviving' becomes 'extant' (I've seen this before).
'on board' indicates putting letters inside (inserted letters are taken on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'extant' inserted into 'ss' is 'SEXTANTS'.
Can you help me to learn more?
(Other definitions for sextants that I've seen before include "Instruments for measuring angular distances" , "Instruments for taking altitudes in navigation and surveying" , "Instruments for taking altitude in navigation and surveying" .)