Hitchin caller regularly observed hard water in winter (6)
I believe the answer is:
icicle
'hard water in winter' is the definition.
(frozen or hard form of water)
'hitchin caller regularly observed' is the wordplay.
'regularly observed' indicates alternate letters (letters selected at regular intervals).
'hitchin'+'caller'='hitchincaller'
The alternate letters of 'hitchincaller' are 'ICICLE'.
(Other definitions for icicle that I've seen before include "Something tapering, seen in freezing weather" , "Winter hanger" , "Frozen water drops" , "A pendant spike of frozen water" , "Hanging piece of frozen liquid" .)