Sand in one's bottle (4)
I believe the answer is:
grit
This clue might be a double definition.
'sand' is the first definition.
(synonyms)
'one's bottle' is the second definition.
'grit' can be an answer for 'bottle' (I've seen this before). I'm not sure about the 'one's' bit.
'in' is the link.
Can you help me to learn more?
(Other definitions for grit that I've seen before include "Indomitability" , "Gumption or small pieces of stone" , "Courage; tiny stones" , "Coarse sandstone - strength of character" , "Small particles of stone or sand" .)