Many insects saw opening close (5)
I believe the answer is:
swarm
'many insects' is the definition.
(eg a swarm of bees)
'saw opening close' is the wordplay.
'saw opening' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'close' becomes 'warm' (similar in meaning).
's'+'warm'='SWARM'
Can you help me to learn more?
(Other definitions for swarm that I've seen before include "Flight of bees" , "Large group of bees" , "Find insect population" , "Large cloud (of insects)" , "Move in large numbers like locusts" .)