Slackened and became loose in a rush (7)
I believe the answer is:
relaxed
'slackened' is the definition.
(I've seen this before)
'loose in a rush' is the wordplay.
'loose' becomes 'lax' (associated in meaning).
'in' indicates putting letters inside.
'a rush' becomes 'reed' (plants found by water).
'lax' going into 'reed' is 'RELAXED'.
'and became' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for relaxed that I've seen before include "Made (a rule) less strict" , "Took it easy, loosened up" , "Easy-going" , "Without strain or anxiety" , "not uptight" .)