Starts to take stock after mostly hating cycling helmets (3,4)

'helmets' is the definition.
(tin hat is a kind of helmet)

'starts to take stock after mostly hating cycling' is the wordplay.
'starts to' suggests taking the first letters.
'after' means one lot of letters go next to another.
'mostly' means to remove the last letter (most but not all of the word).
'cycling' is an anagram indicator.
The initial letters of 'take stock' is 'ts'.
'hating' with its last letter taken away is 'hatin'.
'hatin' with letters rearranged gives 'tinha'.
'ts' put after 'tinha' is 'TIN-HATS'.