Modest shipping engineer liable to tax (6)
I believe the answer is:
tiring
'modest shipping' is the definition.
Although both the answer and definition are present participle verbs, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tiring that I've seen before include "Exhausting or tedious" , "Laborious" , "Growing weary or bored" , "Boring" , "Wearisome" .)