A hindrance to a worker in firm (7)
I believe the answer is:
adamant
'firm' is the definition.
(adamant can mean firm or determined)
'a hindrance to a worker in' is the wordplay.
'a hindrance' becomes 'dam' (both are a kind of blockage).
'to' means one lot of letters go next to another.
'worker' becomes 'ant' (worker is a type of ant).
'in' indicates putting letters inside.
'dam'+'a'='dama'
'dama' going into 'ant' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "Fully determined" , "Resolute; diamond (arch.)" , "Hard and unrelenting" , "Impervious to pleas or persuasion" , "Unswerving" .)