Now and then determined to hold tailless insect (12)
I believe the answer is:
intermittent
'now and then' is the definition.
'determined to hold tailless insect' is the wordplay.
'determined' becomes 'intent' (I've seen this before).
'to hold' indicates putting letters inside.
'tailless' means to remove the last letter.
'insect' becomes 'termite' (termite is a kind of insect).
'termite' with its final letter taken off is 'termit'.
'intent' placed around 'termit' is 'INTERMITTENT'.
(Other definitions for intermittent that I've seen before include "Spasmodic" , "On-and-off" , "Irregular" , "Occurring at intervals" , "Not continuous or steady" .)