Shell trace missing filling station (5)
I believe the answer is:
testa
'shell' is the definition.
(I have seen 'Hard shell' mean 'testa' so perhaps 'shell' could also mean 'testa')
'trace missing filling station' is the wordplay.
I cannot really see how this works, but
'station' could be 'sta' (I have seen 'Short station' mean 'sta' so perhaps 'station' could also mean 'sta') and 'sta' is located in the answer.
The remaining letters 'te' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for testa that I've seen before include "protective coat" , "Botanical term" , "Hard shell" , "Seed-coat" , "cover for a seed" .)