Beginning to grow pale over worry (4)
I believe the answer is:
gnaw
'worry' is the definition.
(thesaurus)
'beginning to grow pale over' is the wordplay.
'beginning to' says to take the initial letters.
'pale' becomes 'wan' ('wan' can be a synonym of 'pale').
'over' shows that the letters should be reversed in order.
The first letter of 'grow' is 'g'.
'wan' written backwards gives 'naw'.
'g'+'naw'='GNAW'
(Other definitions for gnaw that I've seen before include "Bite and wear away" , "Nibble; cause anxiety" , "Nibble like a rodent" , "Prey on one's mind" , "Chew persistently at, say a bone" .)