Clue: 49 to 51, time to be gloomy (6)
Answer: ILLLIT
Explanation: Definition: gloomy
A charade of IL ('49') plus L (50) plus LI ('51') plus T ('time'). A couple of quibbles: 49 in standard Roman numerals is XLIX, and I would have hyphenated the answer – the triple L looks bizarre to me.