Object when gallons going spare (4)
I believe the answer is:
thin
'spare' is the definition.
(similar in meaning)
'object when gallons going' is the wordplay.
I cannot really see how this works, but
'object' could be 'th' (Th is an example) and 'th' is found within the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thin that I've seen before include "Flimsy" , "Fine" , "Having little width" , "Measure" , "Lean; narrow" .)