Citizen's aquatic vessel caught in stream (7)
I believe the answer is:
subject
'citizen's' is the definition.
(I've seen this before)
'aquatic vessel caught in stream' is the wordplay.
'aquatic vessel' becomes 'sub' (submarine is a type of vessel. I am not sure about the 'aquatic' bit.).
'caught' becomes 'c' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'stream' becomes 'jet'.
'c' going within 'jet' is 'ject'.
'sub'+'ject'='SUBJECT'
Can you help me to learn more?
(Other definitions for subject that I've seen before include "Subordinate - part of sentence" , "School topic" , "topic of conversation?" , "Topic being discussed" , "Musical theme" .)