A nerd's worried about beginning of line getting tangled (7)
I believe the answer is:
snarled
'tangled' is the definition.
('snarled' can be similar in meaning to 'tangled')
'a nerd's worried about beginning of line' is the wordplay.
'worried' is an anagram indicator.
'about' indicates putting letters inside.
'beginning of' suggests taking the first letters.
The first letter of 'line' is 'l'.
'a'+'nerds'='anerds'
'anerds' with letters rearranged gives 'snared'.
'snared' enclosing 'l' is 'SNARLED'.
'getting' acts as a link.
(Other definitions for snarled that I've seen before include "Knotted; growled" , "In a twist - showed one's teeth" , "Growled or tangled in knots" .)