Old fellow, confounded pest if not on-topic (3-5)
I believe the answer is:
off-piste
'not on-topic' is the definition.
'old fellow confounded pest if' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'F' (abbreviation - of a society etc.).
'confounded' indicates an anagram.
'pest'+'if'='pestif'
'pestif' with letters rearranged gives 'fpiste'.
'o'+'f'+'fpiste'='OFF-PISTE'
(Other definitions for off-piste that I've seen before include "Away from prepared ski runs" , "Away from the regular runs in skiing" , "On unauthorised ski slope" .)