Singer's ring captured by X-ray at first (5)
I believe the answer is:
tenor
'singer's' is the definition.
(tenor is a kind of singer)
'ring captured by x-ray at first' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'captured by' means one lot of letters goes inside another.
'x' becomes 'ten' (X is the Roman numeral for ten).
'at first' says to take the initial letters.
The first letter of 'ray' is 'r'.
'ten'+'r'='tenr'
'o' placed into 'tenr' is 'TENOR'.
(Other definitions for tenor that I've seen before include "General meaning of an utterance" , "Placido Domingo, say" , "General meaning of a male voice" , "Eg, Luciano Pavarotti" , "Pavarotti, for example" .)