Hold one's breath, drawing in last little bit of air (5)
I believe the answer is:
grasp
'hold one's' is the definition.
'grasp' can be an answer for 'hold' (grasping is a kind of holding). I am unsure of the 'one's' bit.
'breath drawing in last little bit of air' is the wordplay.
'breath' becomes 'gasp' (I've seen this before).
'drawing in' means one lot of letters goes inside another (draw in can mean to contain or trap).
'last little bit of air' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'gasp' going around 'r' is 'GRASP'.
Can you help me to learn more?
(Other definitions for grasp that I've seen before include "Catch on" , "Seize firmly - or understand (5)" , "Hold - understanding" , "Follow - understand - seize" , "Take firm hold" .)