Whistle-blower has mounting hesitation -- shut up or perk up? (7)
I believe the answer is:
refresh
'perk up?' is the definition.
(thesaurus)
'whistle-blower has mounting hesitation shut up' is the wordplay.
'whistle blower' becomes 'ref' (I've seen this in other clues).
'has' means one lot of letters go next to another.
'mounting' shows that the letters should be reversed in order (going up in a down clue).
'hesitation' becomes 'er'.
'shut up' becomes 'sh'.
'er' reversed gives 're'.
'ref'+'re'+'sh'='REFRESH'
'or' acts as a link.
(Other definitions for refresh that I've seen before include "give life again to..." , "Give new strength to" , "brace" , "Give new vigour to" , "Give drink to" .)