Start to need help capturing a water nymph (5)
I believe the answer is:
naiad
'water nymph' is the definition.
(naiad is a kind of water nymph)
'start to need help capturing a' is the wordplay.
'start to' suggests taking the first letters.
'help' becomes 'aid' (synonyms).
'capturing' is an insertion indicator.
The first letter of 'need' is 'n'.
'aid' going around 'a' is 'aiad'.
'n'+'aiad'='NAIAD'
(Other definitions for naiad that I've seen before include "Fabulous spring being" , "Diana (anag.)" , "watery spirit" , "Mythological water-nymph" , "Nymph of lakes, springs and rivers" .)