Recap finding uranium in reservoir (3,2)
I believe the answer is:
sum up
'recap' is the definition.
(I've seen this in another clue)
'uranium in reservoir' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'in' means one lot of letters goes inside another.
'reservoir' becomes 'sump' (sump is a kind of reservoir).
'u' put into 'sump' is 'SUM-UP'.
'finding' acts as a link.
(Other definitions for sum up that I've seen before include "What a judge may do" , "Recapitulate (in court)" , "Of a judge, give the main points of the evidence for jury" , "Summarise" , "(Of a judge) review the evidence" .)