Tommy and I caught by investigator (7,3)
I believe the answer is:
private eye
'investigator' is the definition.
(I've seen this before)
'tommy and i caught' is the wordplay.
'tommy' becomes 'private' (I can't explain this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'caught' indicates a 'sounds like' (homophone) clue (to catch something can mean to hear it).
'i' sounds like 'eye'.
'private'+'eye'='PRIVATE EYE'
'by' is the link.
Can you help me to learn more?
(Other definitions for private eye that I've seen before include "Spade, say" , "supplement to work of police?" , "50 today" , "Freelance detective" , "Periodical" .)