School head nowadays is gloomy (3)
I believe the answer is:
sad
'gloomy' is the definition.
(similar in meaning)
'school head nowadays' is the wordplay.
'head' indicates taking the first letters.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
The initial letter of 'school' is 's'.
's'+'ad'='SAD'
'is' acts as a link.
(Other definitions for sad that I've seen before include "Unfortunate" , "Melancholy, downhearted" , "Melancholy, triste" , "Mournful - sorry" , "Unhappy or melancholy" .)