Solve old jumbo you’ve picked up? (7)
I believe the answer is:
explain
'solve' is the definition.
(thesaurus)
'old jumbo you've picked up?' is the wordplay.
I cannot really see how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found in the answer.
The remaining letters 'plain' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Elucidate - define" , "Describe" , "Resolve" , "Interpret" , "offer clarification" .)