Oscar caught a vet struggling in range (6)
I believe the answer is:
octave
'range' is the definition.
(I have seen 'Musical range ' mean 'octave' so perhaps 'range' could also mean 'octave')
'oscar caught a vet struggling' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'caught' becomes 'c' (cricket abbreviation).
'struggling' is an anagram indicator.
'a'+'vet'='avet'
'avet' is an anagram of 'tave'.
'o'+'c'+'tave'='OCTAVE'
'in' acts as a link.
(Other definitions for octave that I've seen before include "A fence standing" , "Musical interval of eight tones" , "At cove (anag.)" , "Eight consecutive notes on music scale" , "Musical term - set of eight" .)