One group has difficulty concealing evidence of operation (5,5)
I believe the answer is:
pilot light
'evidence of operation' is the definition.
(light used to show a ship is operating)
'one group has difficulty concealing' is the wordplay.
'one' becomes 'i' (Roman numeral).
'group' becomes 'lot'.
'has' means one lot of letters go next to another.
'difficulty' becomes 'plight' (plight is a kind of difficulty).
'concealing' indicates putting letters inside.
'i'+'lot'='ilot'
'ilot' inserted inside 'plight' is 'PILOT LIGHT'.
(Other definitions for pilot light that I've seen before include "what should always be on" , "Airline slightly short-staffed?" , "It inflames" , "It burns all the time" , "ignite the jets" .)